imap/docs/internal.txt

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 * Copyright 1988-2006 University of Washington
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
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	  Documentation of c-client Functions and Interfaces

REVISED: 19 August 1996

				  Credits

     The original version of this document was written by Mark Crispin at
the University of Washington, and described the version of c-client that
supported the IMAP2 (RFC 1176) and IMAP2bis (unpublished) protocols.

     This version is a substantial rewrite of that document, and was
written by Mark Crispin with funding from Sun Microsystems, Incorporated.
Sun's generous support of this work is gratefully acknowledged.


				 Road Map

     This document is organized into the following sections.  Except as
noted, an implementor of an application that uses c-client needs to be
familiar with all of these sections.  Someone who plans to write a new
mailbox driver for c-client (or otherwise modify it) needs to be familiar
with all sections, no exception.

History
	History of how c-client came about.

Overview
	Read this before designing an application that uses c-client.

c-client Structures
	Documentation of several important c-client structs which are
	used in, and returned by, c-client calls.

String Structures
	Documentation of the concept of a "string structure", which
	provides random access to strings without requiring that the
	string be in memory.

c-client Support Functions
	Documentation of support functions for c-client; these deal
	with c-client functionality.

	Only mail_parameters() is of interest to most application
	developers.  Advanced application developers, particularly
	for limited memory systems, may also need to know about the
	readfn_t, mailgets_t, mailcache_t, and tcptimeout_t function
	pointer types, and possibly also the mail_valid_net_parse()
	function.

Mailbox Access Functions
	Documentation of functions which deal with mailboxes;
	listing, subscribing, creating, deleting, renaming, status
	inquiries, opening, and closing mailboxes.

Handle Functions
	Documentation of mail stream handles, which provide protection
	for an advanced application which may have multiple pointers to
	a single mail stream.  If a stream has a handle on it, closing
	the stream does not release its memory, so pointers to it in
	the application remain valid.  Freeing the last handle will free
	the entire stream.

	This is only of interest for advanced application developers.

Message Data Fetching Functions
	Documentation on message data fetching in an open mailbox,
	including parsed representations of RFC-822 and MIME headers
	and message text.  Also how to fetch message attributes (flags,
	internal date, sizes).

Message Status Manipulation Functions
	Documentation on altering message flags in an open mailbox.

Mailbox Searching
	Documentation on searching an open mailbox for messages which
	match certain criteria (e.g. "messages sent July 4 from Jones
	with text `Paris'").

Miscellaneous Mailbox and Message Functions
	Documentation on other operations that would be used by an
	application but that don't fit into any of the above categories.

Date/Time Handling Functions
	Documentation on functions that deal with date/time strings.

	This is only of interest for advanced application developers
	and for implementors of new c-client drivers.

Utility Functions
	Documentation on internal utility functions.

	This is primarily of interest for implementors of new c-client
	drivers, but advanced application developers may also use some
	of these functions.

Data Structure Instantiation/Destruction functions
	Documentation on creating and destroy c-client structures.

	This is primarily of interest for implementors of new c-client
	drivers.  However, application developers will need some of
	these functions to create and destroy structures which are used
	as arguments to various application functions.

Authentication Functions
	Documentation on support for network protocol authentication
	functions.

	This is only of interest for implementors of new c-client
	drivers which deal with authentication mechanisms.

Network Access Functions
	Documentation on creating and destroy c-client structures.

	This is primarily of interest for implementors of new c-client
	drivers which deal with a network.  However, advanced
	application developers may need to use this information if they
	wish to insert their own layer into a network session.

Subscription Management Functions
	Documentation on managing the local (client-based) subscription
	database file.

	This is primarily of interest to advanced application developers.

Miscellaneous Utility Functions
	Documentation on various useful utility functions, such as "make
	a copy of this string."

SMTP Functions
	Documentation on posting email messages via SMTP protocol.

NNTP Functions
	Documentation on posting netnews messages via NNTP protocol.

RFC 822 Support Functions
	Documentation on public RFC-822/MIME functions.

	This is primarily of interest for implementors of new c-client
	drivers and advanced application developers.

Operating System-Dependent Public Interface
	Documentation on OS-dependent functions.  With the exception of
	fs_get(), fs_give(), and fs_resize(), which should be called
	instead of malloc(), free(), and realloc(), these functions are
	primarily of interest for implementors of new c-client drivers.

Main Program Callbacks
	Documentation of functions which the main program must provide
	as callbacks from c-client.

Driver Interface
	Documentation of the driver dispatch vector and the functions
	which a driver must supply.

	This is primarily of interest for implementors of new c-client
	drivers.

Driver Support Functions
	Documentation of support functions which are called by drivers.

	This is primarily of interest for implementors of new c-client
	drivers.
				  History

     The c-client API was originally written by Mark Crispin at Stanford
University as a set of routines to support IMAP and SMTP from a main
program which would handle the user interface.  In its original form, it
was written as the low-level routines that were to be used as part of a
Macintosh client.

     The first IMAP client, MM-D (for "MM on Xerox D machines" -- MM was a
popular DEC-20 mail program) was written in Interlisp for Xerox Lisp
machines.  At that time, there was no name for the embryonic Mac client,
but since it was the first one to be written in C instead of Lisp, it was
given a development name of "C client".  This name became "c-client"
because that is the name of the subdirectory on UNIX where the source files
were stored.

     To exercise the routines, a minimal main program which uses c-client,
mtest, was written.  mtest has subsequently been extended so that it runs
on every platform that c-client is ported.

     The real Mac client, was eventually written by Frank Gilmurrary and
Bill Yeager at Stanford using the autumn 1988 version of c-client and named
"MacMS".  In the winter of 1988-89, Mark Crispin, who had changed jobs to
the University of Washington, developed MS as an MM-like text-based program
for UNIX and MailManager as a GUI-based program for NeXT machines.

     The realization sunk in that this API needed its own name.  As early
as spring 1989, there were at least four programs (mtest, MS, MailManager,
and MacMS) that used it.  The name c-client thus became permanent.

     In its history, c-client has undergone two major redesigns, both by
Mark Crispin who is now on the staff at the University of Washington.

     The first major redesign added the following:
	1) ANSI C calling conventions throughout to assist in function
	   argument type checking.
	2) Vectoring mail access calls through "driver" methods; thus
	   providing transparent access to multiple types of mail
	   stores with the same call.
	3) MIME support.

     The second major redesign was part of the IMAP4 project.  Many
c-client functions were extended with additional arguments and options.
The driver interface was also made simpler, with more work done by
driver-independent code.

			       Overview

     The most important file for the author of an application using the
c-client is mail.h.  mail.h defines several important structures of
data which are passed between the main program and the c-client.
Although some functions (e.g. mail_fetchtext_body()) return the data
fetched, for certain other data items (e.g. flags) you need to get the
data as a structure reference.  mail.h also defines a large number of
useful constants and structures.

     When a function in mail.h exists to reference data, it MUST be
used instead of referencing the structures directly.  This is because
in some cases the data is not actually fetched until a reference (via
the function call) is made.  For example, although the MESSAGECACHE
element for a message can be obtained by indexing the proper cache
element in the stream, there is no guarantee that the item in fact
exists unless mail_fetchstructure_full() is called for that message.
Less costly functions. also exist to create and load a MESSAGECACHE
element.

     The main program will probably also need to include smtp.h,
misc.h, and osdep.h, but this usage should be solely to receive
function prototypes.  Any other definitions in those files should be
considered private to that module.

     Two important predefined symbols are NIL and T.  NIL is any sort
of "false"; T is any sort of "true".  NIL is also used to null-specify
certain optional arguments.

			* * * IMPORTANT * * *

     Any multi-threaded application should test stream->lock prior to
calling any c-client stream functions.  Any attempt to call a
mail_xxx() function while one is already in progress on the same
stream will cause the application to fail in unpredictable ways.

     Note that this check is insufficient in a preemptive-scheduling
multi-tasking application due to the possibility of a timing race.
Such applications must be written so that only one process accesses
the stream, or to have a higher level lock.

     Since MAIL operations will not finish until they are completed, a
single-tasking application does not have to worry about this problem,
except in the callback invoked from MAIL (e.g. mm_exists(), etc.) in which
case the stream is *always* locked.

			  c-client Structures

     c-client has a large number of structures which are used for
multiple functions.  The most important of these are described here.

     The MAILSTREAM structure is used to reference open mailboxes.
Applications may reference the following:

char *mailbox;			mailbox name
unsigned short use;		stream use count, this is incremented
unsigned short sequence;	stream sequence, this is incremented
				 each time a stream is reused (i.e.
				 mail_open() is called to open a
				 different mailbox on this stream)
unsigned int rdonly : 1;	stream is open read-only
unsigned int anonymous : 1;	stream is open with anonymous access
unsigned int halfopen : 1;	stream is half-open; it can be
				 reopened or used for functions that
				 don't need a open mailbox such as
				 mail_create() but no message data
				 can be fetched
unsigned int perm_seen : 1;	Seen flag can be set permanently
unsigned int perm_deleted : 1;	Deleted flag can be set permanently
unsigned int perm_flagged : 1;	Flagged flag can be set permanently
unsigned int perm_answered :1;	Answered flag can be set permanently
unsigned int perm_draft : 1;	Draft flag can be set permanently
unsigned int kwd_create : 1;	new user flags can be created by
				 referencing then in mail_setflag() or
				 mail_clearflag().  Note: this can
				 change during a session (e.g. if
				 there is a limit on the number of
				 keywords), so check after creating a
				 new flag to see if any more can be
				 created before letting the user try
				 to do so
unsigned long perm_user_flags;	corresponding user flags can be set
				 permanently.  This is a bit mask
				 which matches the entries in
				 stream->user_flags[]
unsigned long gensym;		generated unique value.  Always
				 referenced with stream->gensys++
unsigned long nmsgs;		number of messages in current mailbox
unsigned long recent;		number of recent messages in current
				 mailbox
unsigned long uid_validity;	UID validity value; this is used to
				 verify that recorded UIDs match the
				 UIDs that the stream has.  If the
				 mailbox does not have matching UIDs
				 (e.g. the UIDs were lost or not
				 recorded) then the UID validity value
				 will be different
unsigned long uid_last;		highest currently assigned UID in the
				 current mailbox; a new UID will be
				 assigned with ++stream->uid_last
char *user_flags[NUSERFLAGS];	pointers to user flag names in bit
				 order from stream->perm_user_flags or
				 elt->user_flags

     The following MAILSTREAM values are only used internally:

DRIVER *dtb;			dispatch table for this driver
void *local;			pointer to driver local data
unsigned int lock : 1;		stream lock flag (an operation is in
				 progress; used as a bug trap to
				 detect recursion back to c-client
				 from callback routines).
unsigned int debug : 1;		debugging information should be logged
				 via mm_dlog().
unsigned int silent : 1;	don't do main program callbacks on
				 this stream (used when a stream is
				 opened internally)
unsigned int scache : 1;	short caching; don't cache information
				 in memory

     The following MAILSTREAM values are only used by the cache
manager routine (see the documentation about mailcache_t above):

unsigned long cachesize;	size of c-client message cache
union {
  void **c;			to get at the cache in general
  MESSAGECACHE **s;		message cache array
  LONGCACHE **l;		long cache array
} cache;

     The following MAILSTREAM values are for the convenience of
drivers that use short caching and want to be able to garbage collect
any values that they returned:

unsigned long msgno;		message number of `current' message
ENVELOPE *env;			pointer to `current' message envelope
BODY *body;			pointer to `current' message body
char *text;			pointer to `current' text


     The MESSAGECACHE structure (commonly called an "elt" as a
nickname for "cache ELemenT") contains information about messages.
Applications may use the following:

unsigned long msgno;		message number.  If the elt is locked
				 (by elt->lockcount++), then the elt
				 pointer can be stored (e.g. with the
				 data for a window which draws this
				 message) and elt->msgno will change
				 automatically whenever expunges are
				 done so the window will always view
				 the correct message.  If elt->msgno
				 becomes 0, then the message has been
				 expunged, but the elt won't be freed
				 until the elt lock count is
				 decremented (by mail_free_elt()).
unsigned long uid;		message unique ID
unsigned int hours: 5;		internal date hours (0-23)
unsigned int minutes: 6;	internal date minutes (0-59)
unsigned int seconds: 6;	internal date seconds (0-59)
unsigned int zoccident : 1;	non-zero if internal date time zone is
				 west of UTC
unsigned int zhours : 4;	internal date time zone hours from UTC
				 (0-12)
unsigned int zminutes: 6;	internal date time zone minutes (0-59)
unsigned int seen : 1;		message Seen flag
unsigned int deleted : 1;	message Deleted flag
unsigned int flagged : 1; 	message Flagged flag
unsigned int answered : 1;	message Answered glag
unsigned int draft : 1;		message Draft flag
unsigned int valid : 1;		flags are valid in this elt; an elt
				 that was newly created but never
				 loaded with flags won't have this set.
unsigned int recent : 1;	message recent flag
unsigned int searched : 1;	message matches search criteria in
				 most recent mail_search_full() call
unsigned int spare : 1;		reserved for application use
unsigned int spare2 : 1;	reserved for application use
unsigned int spare3 : 1;	reserved for application use
unsigned int lockcount : 8;	non-zero if multiple references to
				 this elt.  Refer to the msgno member
				 for more information.
unsigned int day : 5;		internal date day of month (1-31)
unsigned int month : 4;		internal date month of year (1-12)
unsigned int year : 7;		internal date year since BASEYEAR
				 (currently 1970; was 1969 in older
				 versions so use BASEYEAR instead of
				 having the base year wired in)
unsigned long user_flags;	message user flags; this is a bit mask
				 which matches the entries in
				 stream->user_flags[]
unsigned long rfc822_size;	size of message in octets

     The following MESSAGECACHE values are only used internally by
drivers:

unsigned int sequence : 1;	message is in sequence from either
				 mail_sequence() or mail_uid_sequence()
unsigned long data1;		first data item
unsigned long data2;		second data item
unsigned long data3;		third data item
unsigned long data4;		fourth data item


     The ADDRESS structure is a parsed form of a linked list of RFC 822
addresses.  It contains the following information:

char *personal;			personal name phrase
char *adl;			at-domain-list (also called "source
				 route")
char *mailbox;			mailbox name
char *host;			domain name of mailbox's host
char *error;			error in address from smtp_mail(); if
				 an error is returned from smtp_mail()
				 for one of the recipient addresses
				 the SMTP server's error text for that
				 recipient can be found here.  If it
				 is null then there was no error (or
				 an error was found with a prior
				 recipient
ADDRESS *next;			pointer to next address in list


     The ENVELOPE structure is a parsed form of the RFC 822 header.
Its member names correspond to the RFC 822 field names.  It contains
the following information:

char *remail;			remail header if any
ADDRESS *return_path;		error return address
char *date;			message composition date string
ADDRESS *from;			from address list
ADDRESS *sender;		sender address list
ADDRESS *reply_to;		reply address list
char *subject;			message subject string
ADDRESS *to;			primary recipient list
ADDRESS *cc;			secondary recipient list
ADDRESS *bcc;			blind secondary recipient list
char *in_reply_to;		replied message ID
char *message_id;		message ID
char *newsgroups;		USENET newsgroups
char *followup_to;		USENET reply newsgroups
char *references;		USENET references


     The BODY structure is a parsed form of a linked list of the MIME
structure of a message.  It contains the following information.

unsigned short type;		body primary type code.  This is an
				 index into the body_types vector of
				 body type names.  The following body
				 types are pre-defined:
	TYPETEXT		unformatted text
	TYPEMULTIPART		multiple part
	TYPEMESSAGE		encapsulated message
	TYPEAPPLICATION		application data
	TYPEAUDIO		audio
	TYPEIMAGE		static image (GIF, JPEG, etc.)
	TYPEVIDEO		video
	TYPEOTHER		unknown
				Additional types up to TYPEMAX are
				 dynamically defined if they are
				 encountered by c-client.
unsigned short encoding;	body transfer encoding.  This is an
				 index into the body_encodings vector
				 of body encoding names.  The
				 following body encodings are
				 pre-defined:
	ENC7BIT			7 bit SMTP semantic data
	ENC8BIT			8 bit SMTP semantic data
	ENCBINARY		8 bit binary data
	ENCBASE64		base-64 encoded data
	ENCQUOTEDPRINTABLE	human-readable 8-as-7 bit data
	ENCOTHER		unknown
				Additional encodings up to ENCMAX are
				 dynamically defined if they are
				 encountered by c-client.
char *subtype;			body subtype string
PARAMETER *parameter;		parameter list
char *id;			body content identifier
char *description;		body content description
unsigned char *contents.text;	when composing a message that is NOT
				 of TYPEMULTIPART, non-binary text of
				 the content is stored here.  Note that
				 this happens even when the text is
				 of TYPEMESSAGE.  Text of encoding
				 ENC8BIT may be converted to
				 ENCQUOTEDPRINTABLE when it is sent.
				 This should not be referenced for any
				 other reason; in particular, this is
				 NOT the way for an application to
				 access content data (use
				 mail_fetchbody_full() instead).
BINARY *contents.binary;	when composing a message that is NOT
				 of TYPEMULTIPART, binary content (of
				 encoding ENCBINARY) is stored here.
				 It will be converted to ENCBASE64 when
				 it is sent.
				 This should not be referenced for any
				 other reason; in particular, this is
				 NOT the way for an application to
				 access content data (use
				 mail_fetchbody_full() instead).
PART *contents.part;		for body parts of TYPEMULTIPART, this
				 contains the list of body parts in
				 this multipart
MESSAGE contents.msg;		for body parts of TYPEMESSAGE with
				 subtype "RFC822", this contains the
				 encapsulated message
unsigned long size.lines;	size in lines
unsigned long size.bytes;	size in octets.  This MUST be set when
				composing a message if the encoding is
				ENC8BIT or ENCBINARY.
char *md5;			body content MD5 checksum

     The following BODY information is used only by c-client
internally.  The use of this data is driver-specific and it can not be
relied-upon by applications.

unsigned char *contents.text;	drivers can store a pointer to the
				 body contents as text here.		
unsigned long size.ibytes;	internal size of the body content (prior
				 to newline conversion, etc.) in octets


     The MESSAGE structure is a parsed form of a MESSAGE/RFC822 MIME
body part.  It contains the following information:

ENVELOPE *env;			encapsulated message RFC 822 header
BODY *body;			encapsulated message MIME structure

     The following MESSAGE information is used only by c-client
internally.  The use of this data is driver-specific and it can not be
relied-upon by applications.

char *hdr;			encapsulated message header
unsigned long hdrsize;		message header size
char *text;			message in RFC 822 form
unsigned long offset;		offset of text from header


     The PARAMETER structure is a parsed form of a linked list of
attribute/value pairs.  It contains the following information:

char *attribute;		attribute name
char *value;			value
PARAMETER *next;		next parameter in list


     The PART structure is a parsed form of a linked list of MIME body
parts.  It contains the following information:

BODY body;			body information for this part
PART *next;			next body part

     The following PART information is used only by c-client
internally.  The use of this data is driver-specific and it can not be
relied-upon by applications.

unsigned long offset;		offset from body origin


    The NETMBX structure is a parsed form of a network mailbox name:

char host[NETMAXHOST];		remote host name
char user[NETMAXUSER];		remote user name if specified
char mailbox[NETMAXMBX];	remote mailbox name
char service[NETMAXSRV];	remote service name (IMAP4, NNTP, etc.)
unsigned long port;		TCP/IP port number if specified
unsigned int anoflag : 1;	anonymous access requested
unsigned int dbgflag : 1;	protocol debugging telemetry, via
				 mm_dlog(), requested


     The STRINGLIST structure is a list of strings (which may have
embedded NULs) and their lengths:

char *text;			string text
unsigned long size;		string length
STRINGLIST *next;		next string in list

			  String Structures

     A string structure is analogous to a char*, and is used in some
functions as an input argument.  It represents a string of data in a
way that does not necessarily require the entire string to be in
memory at once.  This is essential for small machines with
highly-restricted memory limits (e.g. DOS).

		       String Structure Access

     To use a string structure, the caller needs to know a string
driver and needs to know the driver-dependent data used by that string
structure.  A simple string driver is mail_string, a string driver
that takes an in-memory char* string as the driver-dependent data.
The DOS port uses string drivers that take a struct holding a file
descriptor and a file offset.  Often the user of a string driver is
the same module that defined it, so usually the programmer knows about
its conventions.

     The following calls are used to access a string structure:

void INIT (STRING *s,STRINGDRIVER *d,void *data,unsigned long size);
	s	pointer to the string structure to be initialized
	d	pointer to the string driver
	data	pointer to driver-dependent data, from which the
		 driver can determine string data
	size	size of the string
 This call initializes the string stucture.


unsigned long SIZE (STRING *s);
	s	pointer to the string structure
 This call returns the number of characters remaining in the string
after the current string character pointer.


char CHR (STRING *s);
	s	pointer to the string structure
 This call returns the character at the current string character
pointer.


char SNX (STRING *s);
	s	pointer to the string structure
 This call returns the character at the current string character
pointer, and increments the string character pointer.


unsigned long GETPOS (STRING *s);
	s	pointer to the string structure
 This returns the value of the current string character pointer.


void SETPOS (STRING *s,unsigned long i);
	s	pointer to the string structure
	i	new string pointer value
 This method sets the string character pointer to the given value.


		      String Structure Internals

     A string structure holds the following data:

void *data;		used by the string driver as it likes
unsigned long data1;	used by the string driver as it likes
unsigned long size;	static, holds the total length of the string
			 from the INIT call
char *chunk;		current chunk of in-memory data; this is used
			 for buffering to avoid unnecessary calls to
			 the string driver's next method.
unsigned long chunksize; size of an in-memory data chunk
unsigned long offset;	position of first character of the chunk in
			 the overall string
char *curpos;		current position; this is what CHR() will
			 access
unsigned long cursize;	number of characters remaining in the current
			 string
STRINGDRIVER *dtb;	the string driver for this string structure


     A string structure is manipulated by a string driver, which has
the following access methods:

void (*init) (STRING *s,void *data,unsigned long size);
	s	pointer to the string structure to be initialized
	data	pointer to driver-dependent data, from which the
		 driver can determine string data
	size	size of the string
 This method initializes the string stucture.  It can use the data,
data1, and chunksize values as it likes.  The remaining values must be
set up as follows:
	size		static, copied from the size argument
	chunk		pointer to a buffer loaded with initial data
	chunksize	size of the buffer
	offset		0
	curpos		copied from chunk
	cursize		copied from chunksize
	dtb		STRINGDRIVER identity pointer


char (*next) (STRING *s);
	s	pointer to the string structure
 This method returns the character at the current string character
pointer, and increments the string character pointer.  This method
is likely to call the setpos method if the desired character is not in
the current chunk.


void (*setpos) (STRING *s,unsigned long i);
	s	pointer to the string structure
	i	new string pointer value
 This method sets the string character pointer to the given value.  If
the pointer is not in the current chunk, then a new chunk is loaded
and the associated values (chunk, offset, curpos, cursize) are
adjusted accordingly.

		      c-client Support Functions


void mail_string_init (STRING *s,void *data,unsigned long size);
char mail_string_next (STRING *s);
void mail_string_setpos (STRING *s,unsigned long i);

     These three functions are the init, next, and setpos string
structure access methods for the build-in mail_string string driver.
mail_string is a basic string driver for a char* string.  See the
documentation below on "String Structures" for more information.


void mail_link (DRIVER *driver);
	driver	pointer to the driver to be added

     This function adds the specified driver to the list of mailbox
drivers.  Initially there are no drivers lunk, so all programs which
intend to use c-client need to have at least one call to this function.

     A function which uses IMAP4 would have a statement such as:
	mail_link (&imapdriver);	/* link in IMAP driver */
early in the program's initialization.  Normally, this is done by the
statement
	#include "linkage.c"
which will include the "system standard driver linkage" defined when
c-client was built.  By using linkage.c instead of explicit mail_link()
calls, you are guaranteed that you will have a consistant linkage among
all software built on this system.


void auth_link (AUTHENTICATOR *auth);
	auth	pointer to the authenticator to be added

     This function adds the specified authenticator to the list of
authenticators.  Initially there are no authenticators lunk.  Normally,
this is done by linkage.c so you don't need to call this routine
explicitly.


void *mail_parameters (MAILSTREAM *stream,long function,void *value);
	stream	stream to poll or NIL
	function function code
	value	new value for function codes that change a parameter

     This function fetches or changes the settings of various c-client
operational parameters depending upon the function.  If the stream is
specified, only the action for the underlying driver for that stream is
taken; however, the scope of the operational parameters is global so
there is generally no reason for the stream argument ever to be
non-NIL.

     The function codes ENABLE_DRIVER and DISABLE_DRIVER take a driver
pointer as a value.  These functions enable and disable mailbox
processing by that driver.  By default, all drivers are enabled.

     The remaining function codes are in a pair named GET_xxx to
fetch an operational parameter and SET_xxx to set the parameter:

 GET_DRIVERS / SET_DRIVERS
	 The list of currently lunk drivers.

 GET_GETS / SET_GETS
	 If non-NIL, points to a function for reading message text.
	Defaults to NIL.
	 This function is called with three arguments; a function
	pointer to a "reading function", a stream for the reading
	function, and a size in octets.  The reading function is
	in turn called with the stream, a size in octets, and a
	pointer to a readin buffer.
	 This function returns with a char* string, which will be
	returned by the mail_fetchheader(), mail_fetchtext(), or
	mail_fetchbody() function which triggered the message text
	reading.
	 The purpose is to permit reading of large strings, without
	requiring an in-memory buffer for the entire string.  The idea
	is that this function can store the data in some form other
	than a char* (e.g. a temporary file) and the main program will
	recognize that it should get the text from there instead of
	from the results from mail_fetch....().
	 This is only supported on DOS and Win16; on other platforms it
	is inconsistent whether or not it works.

 GET_CACHE / SET_CACHE
	 Points to the c-client cache manager function.  Defaults to
	mm_cache().

 GET_SMTPVERBOSE / SET_SMTPVERBOSE
	 If non-NIL, points to a function that accepts a char* string.
	This function is called any time the SMTP routines receive a
	response code less than 100.  The argument is the text of the
	response code

 GET_RFC822OUTPUT / SET_RFC822OUTPUT
	 If non-NIL, points to an alternate rfc822_output() function.
	rfc822_output() will call this function and return instead of
	doing its normal action.  See the description of
	rfc822_output() for more information.	

 GET_USERNAME / SET_USERNAME
	 The logged-in user name.

 GET_HOMEDIR / SET_HOMEDIR
	 The home directory path name.

 GET_LOCALHOST / SET_LOCALHOST
	 The local host name.

 GET_SYSINBOX / SET_SYSINBOX
	 The "system INBOX" (where mail is delivered) path name.

 GET_OPENTIMEOUT / SET_OPENTIMEOUT
	 TCP/IP open timeout in seconds.  Defaults to 0 (system
	default timeout, usually 75 seconds on Unix).
	
 GET_READTIMEOUT / SET_READTIMEOUT
	 TCP/IP read timeout in seconds.  Defaults to 0 (no timeout).

 GET_WRITETIMEOUT / SET_WRITETIMEOUT
	 TCP/IP write timeout in seconds.  Defaults to 0 (no timeout).

 GET_CLOSETIMEOUT / SET_CLOSETIMEOUT
	 TCP/IP close timeout in seconds.  Defaults to 0 (no timeout).

 GET_TIMEOUT / SET_TIMEOUT
	 If non-NIL, points to the function called when a TCP/IP
	timeout occurs.  This function is called with the number of
	seconds since the start of the TCP operation.  If it returns
	non-zero, the TCP/IP operation is continued; if it returns
	non-zero, the TCP/IP connection is aborted.

 GET_RSHTIMEOUT / SET_RSHTIMEOUT
	 rsh connection timeout in seconds.  Defaults to 15 seconds.

 GET_MAXLOGINTRIALS / SET_MAXLOGINTRIALS
	 The maximum number of login attempts permitted in an IMAP or
	POP connection.  Defaults to 3.

 GET_LOOKAHEAD / SET_LOOKAHEAD
	 The number of subsequent envelopes prefetched in IMAP when an
	envelope is fetched.  Defaults to 20.

 GET_IMAPPORT / SET_IMAPPORT
	 The IMAP port number.  Defaults to 143.

 GET_PREFETCH / SET_PREFETCH
	 The number of envelopes prefetched in IMAP from the results
	of a SEARCH.  Defaults to 20.

 GET_CLOSEONERROR / SET_CLOSEONERROR
	 If non-NIL, close an opening IMAP connection if the SELECT
	command fails instead of returning a half-open stream.
	Defaults to NIL.

 GET_POP3PORT / SET_POP3PORT
	 The POP3 port number.  Defaults to 110.

 GET_UIDLOOKAHEAD / SET_UIDLOOKAHEAD
	 The number of UIDs premapped when a message number is
	translated to a UID.  Defaults to 1000.

 GET_MBXPROTECTION / SET_MBXPROTECTION
	 Default file protection for newly created mailboxes.
	Defaults to 0600.

 GET_DIRPROTECTION / SET_DIRPROTECTION
	 Default file protection for newly created directories.
	Defaults to 0700.

 GET_LOCKPROTECTION / SET_LOCKPROTECTION
	 Default file protection for locks.  Defaults to 0666.
	WARNING: don't blithely change this.  If other processes
	can't get access to a lock then they will have trouble in
	locking properly.

 GET_FROMWIDGET / SET_FROMWIDGET
	 If non-NIL, APPEND in the Unix mbox format will insert a
	">" character in front of all lines which begin with the
	string "From ".  If NIL, it will only do so if the entire
	line looks like a message delimiter (that is, the date is
	also in correct format).  Defaults to T.

 GET_NEWSACTIVE / SET_NEWSACTIVE
	 Netnews active file path name.

 GET_NEWSSPOOL / SET_NEWSSPOOL
	 Netnews spool directory path name.

 GET_NEWSRC / SET_NEWSRC
	 Netnews newsgroup reading status file (.newsrc) path name.

 GET_EXTENSION / SET_EXTENSION
	 If non-NIL, points to a string holding the extension for all
	mailbox files.  This is only supported on DOS and Win16.

 GET_DISABLEFCNTLLOCK / SET_DISABLEFCNTLLOCK
	 If non-NIL, disables fcntl() locking on SVR4.  This is done
	if fcntl() tends to hang for no good reason.  Now that the
	fcntl() code checks for NFS files and no-ops the locking,
	this problem usually doesn't happen much any more.  Defaults
	to NIL.

 GET_LOCKEACCESERROR / SET_LOCKEACCESERROR
	 If non-NIL, give a warning if an attempt to create a .lock
	file gets an EACCES ("Permission denied") error.  This usually
	means that somebody protected the system inbox directory (e.g.
	/var/mail) instead of making it public-write with the sticky
	bit.  Defaults to non-NIL, since this is usually bad news.

 GET_LISTMAXLEVEL / SET_LISTMAXLEVEL
	 The maximum depth of recusion that LIST will go on a *
	wildcard.  Defaults to 20.

 GET_ANONYMOUSHOME / SET_ANONYMOUSHOME
	 The anonymous use home directory name.


typedef long (*readfn_t) (void *stream,unsigned long size,char *buffer);
	stream	a designator suitable
	size	a number of octets to read
	buffer	a buffer of at least size octets for readin

     This function reads the given number of octets into the buffer,
using the given stream.  What sort of object the stream is depends upon
the function and its caller, so you must make sure that the readfn is
suitable for the caller's purpose.  Common uses include support of the
mailgets function (see below) and of reading from local files on systems
with limited address space.


typedef char *(*mailgets_t) (readfn_t f,void *stream,unsigned long size);
	f	the readfn to use
	stream	stream argument for the readfn
	size	total number of octets to read

     This is the argument to the SET_GETS mail_parameter() call.  This
function must read size octets from the stream, using the readfn f.  It
may call f multiple times to accomplish this; this will read the data in
a serial fashion.  So, for example, if size is a megabyte and there is
only 4K of available buffer space, it can call f 256 times to satisfy
the request.  There is no way to back up in the reading, so any
processing or saving of the data must be done when it is read.

     The function mm_gets() in mail.c is a sample mailgets function; it
reads the first MAXMESSAGESIZE of data into memory and discards the
rest.


typedef void *(*mailcache_t) (MAILSTREAM *stream,unsigned long msgno,long op);
	stream	stream to cache manage
	msgno	message to cache manage in the stream
	op	cache management operation

     This function manages the c-client cache.  Normally, a program will
use the default c-client cache manager routine mm_cache().  However, a
main program may want to supply its own cache manager, e.g. it may want
to store the data on a disk file instead of in memory on DOS and Win16
where memory is tight.

     If you write your own cache manager, you need to examine the
default mm_cache() manager closely, as well as paying close attention to
what goes into an elt (a MESSAGECACHE element).  It is highly likely
that if you roll elts out to disk, you will want to set stream->scache
and *NOT* use long elts (because long elts have ENVELOPE and BODY
pointers that you would have to know how to write to disk and read back).

     The cache management functions are one of the following:

 CH_INIT	 Initialize the entire cache for the stream.  This is
		called only when creating a new stream or when freeing
		it.  The msgno argument is ignored.

 CH_SIZE	 Make sure that the cache is at least large enough to
		support msgno.  This is a request to grow the cache if
		necessary, not shrink it.

 CH_MAKELELT	 Return a long elt for msgno, creating it if necessary.
		This is the underlying support function for mail_lelt().

 CH_LELT	 Return the long elt for msgno, or NIL if it does not
		already exist.

 CH_MAKEELT	 Return an elt for msgno, creating it if necessary.
		This is the underlying support function for mail_elt().

 CH_ELT		 Return the elt for msgno, or NIL if it does not already
		exist.

 CH_FREE	 Free the [l]elt for msgno.

 CH_EXPUNGE	 Free the [l]elt for msgno, and reclaim its position.
		All subsequent elts are renumbered with their elt->msgno
		decremented by 1.  [Hence msgno+1 becomes msgno, etc.]
		This supports message expunging from the cache.


typedef long (*tcptimeout_t) (long time);
	time	total time spent since TCP operation started

     This function is called when a TCP operation times out.  It is set
by the SET_TIMEOUT mail_parameter().  The function can return non-zero
to continue the TCP operation (e.g. after outputting a "do you still
want to wait" prompt) or zero if it wants the TCP operation to abort and
close.  If the TCP operation aborts, it will likely cause the upper
level IMAP, SMTP, etc. stream to abort and close as well.


DRIVER *mail_valid (MAILSTREAM *stream,char *mailbox,char *purpose);
	stream	if non-NIL, stream to use for validation
	mailbox	mailbox name to validate
	purpose	filled in as xxx in "Can't xxx" in error messages

     This function validates the given mailbox name.  It successful, it
returns the driver that can open that name if successful, otherwise it
returns NIL.  If stream is non-NIL, the mailbox name must be valid for
the type of mailbox associated with that stream (e.g. an NNTP name can
not be used with an IMAP stream).  If purpose is non-NIL, an error
message is passed via mm_log() when an error occurs.


DRIVER *mail_valid_net (char *name,DRIVER *drv,char *host,char *mailbox);
	name	mailbox name to validate
	drv	driver name to validate against
	host	buffer to return host name if non-NIL
	mailbox	buffer to return remote mailbox name if non-NIL

     This function is an alternative to mail_valid_net_parse().  It
validates the given mailbox name as a network name and makes sure that
its service name is the same as the driver in drv.  If successful, it
returns drv, and copies the host and mailbox strings as needed.
Otherwise it returns NIL.


long mail_valid_net_parse (char *name,NETMBX *mb);
	name	mailbox name to parse
	mb	pointer to NETMBX structure to return

     This function parses a network mailbox name.  If the name is a
network mailbox name, it returns non-NIL, with the NETMBX structure
loaded with the results form the parse.

		       Mailbox Access Functions

void mail_list (MAILSTREAM *stream,char *ref,char *pat);
void mail_scan (MAILSTREAM *stream,char *ref,char *pat,char *contents);
	stream	if non-NIL, stream to use
	ref	mailbox reference string
	pat	mailbox pattern string
	contents contents to search

     This function returns a list of mailboxes via the mm_list()
callback.  The reference is applied to the pattern in an implementation
dependent fashion, and the resulting string is used to search for
matching mailbox names.  "*" is a wildcard which matches zero or more
characters; "%" is a variant which does not descend a hierarchy level.
Read the IMAP specification for more information.

     mail_scan() is a variant which takes a string to search for in the
text of the mailbox.  The string is a free-text string, without regard
for message boundaries, and thus the choice of strings must be made
with care.


void mail_lsub (MAILSTREAM *stream,char *ref,char *pat);
	stream	if non-NIL, stream to use
	ref	mailbox reference string
	pat	mailbox pattern string

     This function returns a list of subscribed mailboxes via the
mm_lsub() callback.  The reference is applied to the pattern in an
implementation dependent fashion, and the resulting string is used to
search for matching mailbox names in the subscription list.  "*" is a
wildcard which matches zero or more characters; "%" is a variant which
does not descend a hierarchy level.  Read the IMAP specification for
more information.


long mail_subscribe (MAILSTREAM *stream,char *mailbox);
	stream	if non-NIL, stream to use
	mailbox	mailbox name

     This function adds the given name to the subscription list.  It
returns T if successful, NIL if unsuccessful.  If unsuccessful, an
error message is returned via the mm_log() callback.


long mail_unsubscribe (MAILSTREAM *stream,char *mailbox);
	stream	if non-NIL, stream to use
	mailbox	mailbox name

     This function removes the given name from the subscription list.
It returns T if successful, NIL if unsuccessful.  If unsuccessful, an
error message is returned via the mm_log() callback.


long mail_create (MAILSTREAM *stream,char *mailbox);
	stream	if non-NIL, stream to use
	mailbox	mailbox name

     This function creates a mailbox with the given name.  It returns T
if successful, NIL if unsuccessful.  If unsuccessful, an error message
is returned via the mm_log() callback.

     It is an error to create INBOX or a mailbox name which already
exists.


long mail_delete (MAILSTREAM *stream,char *mailbox);
	stream	if non-NIL, stream to use
	mailbox	mailbox name

     This function deletes the named mailbox.  It returns T if
successful, NIL if unsuccessful.  If unsuccessful, an error message is
returned via the mm_log() callback.

     It is an error to delete INBOX or a mailbox name which does not
already exist.


long mail_rename (MAILSTREAM *stream,char *old,char *newname);
	stream	if non-NIL, stream to use
	old	existing mailbox name
	newname	new (not yet existing) mailbox name

     This function renames the old mailbox to the new mailbox name.
It returns T if successful, NIL if unsuccessful.  If unsuccessful, an
error message is returned via the mm_log() callback.

     It is an error to reanme a mailbox that does not exist, or rename
a mailbox to a name that already exists.  It is permitted to rename
INBOX; a new empty INBOX is created in its place.


long mail_status (MAILSTREAM *stream,char *mbx,long flags);
	stream	if non-NIL, stream to use
	mbx	mailbox name
	flags	option flags

     This function returns the status of the given mailbox name via the
mm_status() callback.  It returns T if successful, NIL if unsuccessful.
If unsuccessful, an error message is returned via the mm_log()
callback.

     The options are a bit mask with one or more of the following,
indicating the data which should be returned.
	SA_MESSAGES	number of messages in the mailbox
	SA_RECENT	number of recent messages in the mailbox
	SA_UNSEEN	number of unseen messages in the mailbox
	SA_UIDNEXT	next UID value to be assigned
	SA_UIDVALIDITY	UID validity value

     Note that, depending upon implementation, some of these values may
be more costly to get than others.  For example, calculating the
number of unseen messages may require opening the mailbox and scanning
all of the message flags.  A mail_status() call should thus be used
with option flags specifying only the data that is actually needed.


MAILSTREAM *mail_open (MAILSTREAM *oldstream,char *name,long options);
	oldstream if non-NIL, stream to recycle
	name	mailbox name to open
	options	option flags.

     This function opens the mailbox and if successful returns a stream
suitable for use by the other MAIL functions.

     If oldstream is non-NIL, an attempt is made to reuse oldstream as
the stream for this mailbox; this is useful when you want to open
another mailbox to the same IMAP or NNTP server without having to open
a new connection.  Doing this will close the previously open mailbox.

     The options are a bit mask with one or more of the following:
	OP_DEBUG	Log IMAP protocol telemetry through mm_debug()
	OP_READONLY	Open mailbox read-only.
	OP_ANONYMOUS	Don't use or update a .newsrc file for news.
	OP_SHORTCACHE	Don't cache envelopes or body structures
	OP_SILENT	Don't pass mailbox events (internal use only)
	OP_PROTOTYPE	Return the "prototype stream" for the driver
			 associated with this mailbox instead of
			 opening the stream
	OP_HALFOPEN	For IMAP and NNTP names, open a connection
			 to the server but don't open a mailbox.
	OP_EXPUNGE	Silently expunge the oldstream before recycling

 NIL is returned if this function fails for any reason.


MAILSTREAM *mail_close (MAILSTREAM *stream);
MAILSTREAM *mail_close_full (MAILSTREAM *stream,long options);
	stream	stream to close
	options	option flags
     This function closes the MAIL stream and frees all resources
associated with it that it may have created (subject to any handles
existing).

     The options for mail_close_full() are a bit mask with one or more
of the following:
	CL_EXPUNGE	Silently expunge before closing

     This function always returns NIL, so it can be used as:
	stream = mail_close (stream);

			   Handle Functions

     Handles are used when an entity that wishes to access the stream
may survive the stream without knowing that it outlived it.  For
example, an object reading a message may have a handle to a stream,
but the message selection object that spawned it (and which owns the
stream) may have gone away.  A stream can be closed or recycled while
handles are pointing at it, but it is not completely freed until all
handles are gone.  A stream may have an arbitrary number of handles.


MAILHANDLE *mail_makehandle (MAILSTREAM *stream);
	stream	stream to make handle to

     This function creates and returns a handle to the stream.


void mail_free_handle (MAILHANDLE **handle);
	handle	pointer to handle to release

     This function frees the handle and notifies the stream that it has
one fewer handle.  If this is the last handle on the stream and the
stream has been closed, then the stream is freed.


MAILSTREAM *mail_stream (MAILHANDLE *handle);
	handle	handle to look up

     This function returns the stream associated with the handle if and
only if the stream still represents the same MAIL connection associated
with the handle.  Otherwise, NIL is returned (meaning that there is no
active stream associated with this handle).

		    Message Data Fetching Functions

[Note!!  There is an important difference between a "sequence" and a
 "msgno".  A sequence is a string representing one or more messages in
 IMAP4-style sequence format ("n", "n:m", or combination of these
 delimited by commas), whereas a msgno is an int representing a single
 message.] 

void mail_fetchfast (MAILSTREAM *stream,char *sequence);
void mail_fetchfast_full (MAILSTREAM *stream,char *sequence,long flags);
	stream	stream to fetch on
	sequence IMAP-format set of message sequence numbers
	flags	option flags

     This function causes a cache load of all the "fast" information
(internal date, RFC 822 size, and flags) for the given sequence.  Since
all this information is also fetched by mail_fetchstructure(), this
function is generally not used unless the OP_SHORTCACHE option in the
mail_open() call is used.

     The options for mail_fetchfast_full() are a bit mask with one or
more of the following:
	FT_UID		The sequence argument contains UIDs instead of
			 sequence numbers


void mail_fetchflags (MAILSTREAM *stream,char *sequence);
void mail_fetchflags_full (MAILSTREAM *stream,char *sequence,long flags);

     This function causes a fetch of the flags for the given sequence.
This main reason for using this function is to update the flags in the
local cache in case some other process changed the flags (multiple
simultaneous write access is allowed to the flags) as part of a "check
entire mailbox" (as opposed to "check for new messages") operation.

 The options for mail_fetchflags_full() are a bit mask with one or more
of the following:
	FT_UID		The sequence argument contains UIDs instead of
			 sequence numbers


ENVELOPE *mail_fetchenvelope (MAILSTREAM *stream,unsigned long msgno);
ENVELOPE *mail_fetchstructure (MAILSTREAM *stream,unsigned long msgno,
			       BODY **body);
ENVELOPE *mail_fetchstructure_full (MAILSTREAM *stream,unsigned long msgno,
				    BODY **body,long flags);
	stream	stream to fetch on
	msgno	message sequence number
	body	pointer to where to return BODY structure if non-NIL
	flags	option flags
     This function causes a fetch of all the structured information
(envelope, internal date, RFC 822 size, flags, and body structure) for
the given msgno and, in the case of IMAP, up to MAPLOOKAHEAD (a
parameter in IMAP2.H) subsequent messages which are not yet in the
cache.  No fetch is done if the envelope for the given msgno is already
in the cache.  The ENVELOPE and the BODY for this msgno is returned.
It is possible for the BODY to be NIL, in which case no information is
available about the structure of the message body.

     The options for mail_fetchstructure_full() are a bit mask with one
or more of the following:
	FT_UID		The msgno argument is a UID 

     This is the primary function for fetching non-text information
about messages, and should be called before any attempt to reference
cache information about this message via mail_elt().


char *mail_fetchheader (MAILSTREAM *stream,unsigned long msgno);
char *mail_fetchheader_full (MAILSTREAM *stream,unsigned long msgno,
			     STRINGLIST *lines,unsigned long *len,long flags);
	stream	stream to fetch on
	msgno	message sequence number
	lines	list of header lines to fetch
	len	returned length in octets
	flags	option flags

     This function causes a fetch of the complete, unfiltered RFC 822
format header of the specified message as a text string and returns
that text string.

     If the lines argument is non-NIL, it contains a list of header
field names to use in subsetting the header text.  Only those lines
which have that header field name are returned, unless FT_NOT is set in
which case only those lines which do not have that header field name
are returned.

     If the len argument is non-NIL, it holds a pointer in which the
length of the string in octets is returned.  This is useful in cases
where there may be an embedded null in the string.

     This function always returns a valid string pointer; if no header
exists or if it can not be fetched (e.g. by a deceased IMAP stream) an
empty string is returned.

     The options for mail_fetchheader_full() are a bit mask with one or
more of the following:
	FT_UID		The msgno argument is a UID 
	FT_NOT		The returned header lines are those that are
			 not in the lines argument
	FT_INTERNAL	The return string is in "internal" format,
			 without any attempt to canonicalize to CRLF
			  newlines
	FT_PREFETCHTEXT	The RFC822.TEXT should be pre-fetched at the
			 same time.  This avoids an extra RTT on an
			 IMAP connection if a full message text is
			 desired (e.g. in a "save to local file"
			 operation)
		 

char *mail_fetchtext (MAILSTREAM *stream,unsigned long msgno);
char *mail_fetchtext_full (MAILSTREAM *stream,unsigned long msgno,
			   unsigned long *len,long flags);
	stream	stream to fetch on
	msgno	message sequence number
	len	returned length in octets
	flags	option flags

     This function causes a fetch of the non-header text of the
specified message as a text string and returns that text string.  No
attempt is made to segregate individual body parts.

     If the len argument is non-NIL, it holds a pointer in which the
length of the string in octets is returned.  This is useful in cases
where there may be an embedded null in the string.

     This function always returns a valid string pointer; if no header
exists or if it can not be fetched (e.g. by a deceased IMAP stream) an
empty string is returned.

      The options for mail_fetchtext_full() are a bit mask with one or
more of the following:
	FT_UID		The msgno argument is a UID 
	FT_PEEK		Do not set the \Seen flag if it not already set
	FT_INTERNAL	The return string is in "internal" format,
			 without any attempt to canonicalize to CRLF
			  newlines


char *mail_fetchbody (MAILSTREAM *stream,unsigned long msgno,char *sec,
		      unsigned long *len);
char *mail_fetchbody_full (MAILSTREAM *stream,unsigned long msgno,char *sec,
			   unsigned long *len,long flags);
	stream	stream to fetch on
	msgno	message sequence number
	sec	section specifier
	len	returned length in octets
	flags	option flags

      This function causes a fetch of the particular section of the
body of the specified message as a text string and returns that text
string.  The section specification is a string of integers delimited by
period which index into a body part list as per the IMAP4
specification.  Body parts are not decoded by this function; see
rfc822_base64() and rfc822_quotedprintable().

     If the len argument is non-NIL, it holds a pointer in which the
length of the string in octets is returned.  This is useful in cases
where there may be an embedded null in the string.

      This function may return NIL on error.

      The options for mail_fetchbody_full() are a bit mask with one or
more of the following:
	FT_UID		The msgno argument is a UID 
	FT_PEEK		Do not set the \Seen flag if it not already set
	FT_INTERNAL	The return string is in "internal" format,
			 without any attempt to canonicalize to CRLF
			  newlines


unsigned long mail_uid (MAILSTREAM *stream,unsigned long msgno);
	stream	stream to fetch on
	msgno	message sequence number

      This function returns the UID for the given message sequence
number.


void mail_fetchfrom (char *s,MAILSTREAM *stream,unsigned long msgno,
		     long length);
	s	destination string
	stream	stream to fetch on
	msgno	message sequence number
	length	maximum field length

     This function writes a "from" string of the specified length for
the specified message, suitable for display to the user in a menu line,
into the string pointed to by s.

      If the personal name of the first address in the envelope's from
item is non-NIL, it is used; otherwise a string is created by appending
the mailbox of the first address, an "@", and the host of the first
address.  The string is trimmed or padded with trailing spaces as
necessary to make its length match the length argument.


void mail_fetchsubject (char *s,MAILSTREAM *stream,unsigned long msgno,
			long length);
	s	destination string
	stream	stream to fetch on
	msgno	message sequence number
	length	maximum field length

      This function returns a "subject" string of the specified length
for the specified message, suitable for display to the user in a menu
line.

       The envelope's subject item is copied and trimmed as necessary
to make its length be no more what the caller requested.  Unlike
mail_fetchfrom(), this function can return a string of shorter length
than what the caller requested.


LONGCACHE *mail_lelt (MAILSTREAM *stream,unsigned long msgno);
MESSAGECACHE *mail_elt (MAILSTREAM *stream,unsigned long msgno);
	stream	stream to access
	msgno	message sequence number

     This function returns the cache entry for the specified message.
Although it will create a cache entry if it does not already exist,
that functionality is for internal use only.  This function should
never be called without having first called mail_fetchfast() or
mail_fetchstructure() on the message first.

     A cache entry holds the internal date/time, flags, and RFC 822
size of a message.  It holds other data as well, but that is for
internal use only.

     mail_lelt() is a variant that returns a `long' cache entry, which
consists of an cache entry (as a structure, not a pointer), an envelope
pointer, and a body pointer.  This is used in conjunction with the elt
lock count functionality, to allow an application to associate the
cached envelope and body of a message with an open window even if the
message is subsequently expunged or if the stream is closed.

     Unless your application wants to look at cached envelopes and
bodies even after the message is expunged or the stream is closed, it
should not use mail_lelt().  Instead, it should use a returned elt from
mail_elt() and use the elt->msgsno as the argument to
mail_fetchstructure().

	BEWARE: the behavior of mail_lelt() is undefined if the
	stream is open with OP_SHORTCACHE.  mail_lelt() is extremely
	special purpose, and should only be used in sophisticated
	special purpose applications after discussing its use with
	the c-client author.  If you think you need this function,
	you are probably mistaken.  In almost all cases, you should
	use mail_elt() and mail_fetchstructure() instead.

		 Message Status Manipulation Functions

void mail_setflag (MAILSTREAM *stream,char *sequence,char *flag);
void mail_setflag_full (MAILSTREAM *stream,char *sequence,char *flag,
			long flags);
	stream	stream to use
	sequence IMAP-format set of message sequence numbers
	flag	IMAP-format flag string
	flags	option flags

    This function causes a store to add the specified flag to the flags
set for the messages in the specified sequence.  If there is any
problem in setting flags, a message will be passed to the application
via the mm_log() facility.

     The options for mail_setflag_full() are a bit mask with one or
more of the following:
	ST_UID		The sequence argument contains UIDs instead of
			 sequence numbers
	ST_SILENT	Do not update the local cache with the new
			 value of the flags.  This is useful to save
			 network bandwidth, at the cost of invalidating
			 the cache.


void mail_clearflag (MAILSTREAM *stream,char *sequence,char *flag);
void mail_clearflag_full (MAILSTREAM *stream,char *sequence,char *flag,
			  long flags);
	stream	stream to use
	sequence IMAP-format set of message sequence numbers
	flag	IMAP-format flag string
	flags	option flags

     This function causes a store to delete the specified flag from the
flags set for the messages in the specified sequence.  If there is any
problem in clearing flags, a message will be passed to the application
via the mm_log() facility.

     The options for mail_setflag_full() are a bit mask with one or
more of the following:
	ST_UID		The sequence argument contains UIDs instead of
			 sequence numbers
	ST_SILENT	Do not update the local cache with the new
			 value of the flags.  This is useful to save
			 network bandwidth, at the cost of invalidating
			 the cache.

			   Mailbox Searching

void mail_search (MAILSTREAM *stream,char *criteria);
void mail_search_full (MAILSTREAM *stream,char *charset,SEARCHPGM *pgm,
		       long flags);
	stream	stream to search
	charset	MIME character set to use when searching strings
	pgm	search program
	flags	option flags

     This function causes a mailbox search, using the given MIME
charset (NIL means the default, US-ASCII) and the given search program.
A search program is a structure that holds the following data:

SEARCHSET *msgno;	a set of message sequence numbers
SEARCHSET *uid;		a set of unique identifiers
SEARCHOR *or;		OR result of two search programs
SEARCHPGMLIST *not;	AND result of list of NOT'ed search programs
SEARCHHEADER *header;	message headers
STRINGLIST *bcc;	string(s) appear in bcc list
STRINGLIST *body;	string(s) appear in message body text
STRINGLIST *cc;		string(s) appear in cc list
STRINGLIST *from;	string(s) appear in from
STRINGLIST *keyword;	user flag string(s) set
STRINGLIST *unkeyword;	user flag strings() not set
STRINGLIST *subject;	string(s) appear in subject
STRINGLIST *text;	string(s) appear in message header or body
STRINGLIST *to;		string(s) appear in to list
unsigned long larger;	larger than this many octets
unsigned long smaller;	smaller than this many octes
	The following dates are in form:
		((year - BASEYEAR) << 9) + (month << 5) + day
unsigned short sentbefore;
			sent before this date
unsigned short senton;	sent on this date
unsigned short sentsince;
			sent since this date
unsigned short before;	received before this date
unsigned short on;	received on this date
unsigned short since;	received since this date
unsigned int answered : 1;
			message answered
unsigned int unanswered : 1;
			message not answered
unsigned int deleted : 1;
			message deleted
unsigned int undeleted : 1;
			message not deleted
unsigned int draft : 1;	message is a draft
unsigned int undraft : 1;
			message is not a draft
unsigned int flagged : 1;
			message flagged as urgent
unsigned int unflagged : 1;
			message not flagged as urgent
unsigned int recent : 1;
			message recent since last parse of mailbox
unsigned int old : 1;	message not recent since last parse of mailbox
unsigned int seen : 1;	message read
unsigned int unseen : 1;
			message not read

     The following auxillary structures are used by search programs:
	SEARCHHEADER:	header line searching
char *line;		header line field name
char *text;		text header line
SEARCHHEADER *next;	next SEARCHHEADER in list (AND'ed)

	SEARCHSET:	message number set
unsigned long first;	first number in set
unsigned long last;	if non-zero, last number in set
SEARCHSET *next;	next SEARCHSET in list (AND'ed)

	SEARCHOR:	two search programs, OR'ed together
SEARCHPGM *first;	first program
SEARCHPGM *second;	second program
SEARCHOR *next;		next SEARCHOR in list

	SEARCHPGMLIST:	list of search programs
SEARCHPGM *pgm;		search program (AND'd with others in list)
SEARCHPGMLIST *next;	next SEARCHPGM in list

     mail_search(), the older interface, accepts a search criteria
argument as a character string in IMAP2 (RFC-1176) format.  Do not try
to use any IMAP4 search criteria with this interface.

     The application's mm_searched() function is called for each
message that matches the search criteria.  In addition, after the
search is completed, the "fast" information (see mail_fetchfast_full()
and envelopes of the searched messages are fetched (this is called
pre-fetching).

     If there is any problem in searching, a message will be passed to
the application via the mm_log() facility.

     The flags for mail_search_full() are a bit mask with one or more
of the following:
	SE_UID		Return UIDs instead of sequence numbers
	SE_FREE		Return the search program to free storage after
			 finishing
	SE_NOPREFETCH	Don't prefetch searched messages.


unsigned long *mail_sort (MAILSTREAM *stream,char *charset,SEARCHPGM *spg,
			  SORTPGM *pgm,long flags);
	stream	stream to sort
	charset	MIME character set to use when sorting strings
	spg	search program
	pgm	sort program
	flags	option flags


     This function is a variant of mail_search_full().  It accepts an
additional argument, a sort program, which specifies one or more sort
rules to be applied to the result.  If the searching and sorting are
successful, it returns a 0-terminated vector of message sequence
numbers (or UIDs if SE_UID is set).  This vector is created out of
free storage, and must be freed with fs_give() when finished with it.

     A sort program is a structure that holds the following data:
unsigned int reverse : 1;
			reverse sorting of this key
short function;		sort rule, one of the following:
		SORTDATE	message Date
		SORTARRIVAL	arrival date
		SORTFROM	mailbox in first From address
		SORTSUBJECT	message Subject
		SORTTO		mailbox in first To address 
		SORTCC		mailbox in first cc address 
		SORTSIZE	size of message in octets
SORTPGM *next;		next sort program to be applied if two or more
			 messages collate identically with this rule

     The flags for mail_search_full() are a bit mask with one or more
of the following:
	SE_UID		Return UIDs instead of sequence numbers
	SE_FREE		Return the search program to free storage after
			 finishing
	SE_NOPREFETCH	Don't prefetch searched messages.
	SO_FREE		Return the sort program to free storage after
			 finishing

	      Miscellaneous Mailbox and Message Functions

long mail_ping (MAILSTREAM *stream);
	stream	string to ping

     The function pings the stream to see if it is still active.  It may
discover new mail; this is the preferred method for a periodic "new mail
check" as well as a "keep alive" for servers which have an inactivity
timeout.  It returns T if the stream is still alive, NIL otherwise.

     If new mail is found, the application's mm_exists() function is
called with the newly-determined number of messages in the mailbox.


void mail_check (MAILSTREAM *stream);
	stream	stream to checkpoint

      This function causes a mailstore-defined checkpoint of the
mailbox.  This may include such things as a writeback to disk, a check
for flag changes in a shared mailbox, etc.  It is not a "check for new
mail"; mail_ping() performs this function (as potentially does any other
function).  The status of the check is passed to the application via the
mm_log() facility.


void mail_expunge (MAILSTREAM *stream);
	stream	string to expunge

     This function causes an expunge (permanent removal of messages
which are marked as deleted) of the mailbox.  The application's
mm_expunged() function is called for each message that has been
expunged.  The application's mm_exists() function is called at the start
and end of the expunge to ensure synchronization.  The status of the
expunge is passed to the application via the mm_log() facility.

      Note that the decrementing of msgno's for subsequent messages
happens immediately; for example, if three consequtive messages starting
at msgno 5 are expunged, mm_expunged() will be called with a msgno of 5
three times.


long mail_copy (MAILSTREAM *stream,char *sequence,char *mailbox);
long mail_move (MAILSTREAM *stream,char *sequence,char *mailbox);
long mail_copy_full (MAILSTREAM *stream,char *sequence,char *mailbox,
		     long options);
	stream	stream to copy
	sequence IMAP-format set of message numbers
	mailbox	destination mailbox name
	options	option flags

     This function causes the messages in the specified sequence to be
copied to the specified mailbox.  T is returned if the copy is
successful.  mail_move() is equivalent to setting CP_MOVE in the options.

     If there is any problem in copying, a message will be passed to
the application via the mm_log() facility and the function returns NIL.
No copying is actually done in this case.

      Note that the mailbox must be on the same host as the stream and
is a mailbox of the type of the source mailbox only.

     The flags for mail_search_full() are a bit mask with one or more
of the following:
	CP_UID		The sequence argument contains UIDs instead of
			 sequence numbers
	CP_MOVE		Delete the messages from the current mailbox
			 after copying to the destination.


long mail_append (MAILSTREAM *stream,char *mailbox,STRING *message);
long mail_append_full (MAILSTREAM *stream,char *mailbox,char *flags,char *date,
		       STRING *message);
	stream	stream to use if non-NIL (in the IMAP case)
	mailbox	destination mailbox name
	flags	flags to set on message if non-NIL
	date	internal date (received date) to set on message if non-NIL
	message	string structure of message to write

     This function writes the message in the string structure to the
destination mailbox, along with the flags and date if specified.  This
is useful in those cases where you can't use mail_copy(), e.g. when
copying from one server to another; you can always fetch the message
and then mail_append() it to the destination.  It may also be useful
for maintaining an outbox of your outgoing mail.


void mail_gc (MAILSTREAM *stream,long gcflags);
	stream	stream to GC if non-NIL (else GC's all streams)
	flags	option flags

      This function garbage collects (purges) the cache of entries of
a specific type.  Some drivers do not allow purging of particular
cache types, and an attempt to do so is ignored.

      The flags for mail_gc() are a bit mask with one or more of the
following:
	GC_ELT		message cache elements
	GC_ENV		ENVELOPEs and BODYs
	GC_TEXTS	cached texts

		     Date/Time Handling Functions


char *mail_date (char *string,MESSAGECACHE *elt);
	string	destination string
	elt	message cache element containing date

      This function accepts a message cache element that contains date
information, and writes an IMAP-4 date string, that is, one in form:
	dd-mmm-yyyy hh:mm:ss +zzzz
based upon the data in the elt.  The destination string must be large
enough to hold this string.


char *mail_cdate (char *string,MESSAGECACHE *elt);
	string	destination string
	elt	message cache element containing date

      This function accepts a message cache element that contains date
information, and writes a ctime() format date string, that is, one in
form:
	www mmm dd hh:mm:ss yyyy\n
based upon the data in the elt.  The destination string must be large
enough to hold this string.


long mail_parse_date (MESSAGECACHE *elt,char *string);
	elt	message cache element to store parsed date
	string	source date string

      This function parses the date/time stored in the given string,
in format:
	[www,] date [[hh:mm[:ss][-zzz| +zzzz]
where the date can be any of:
	mm/dd/yy, mm/dd/yyyy, dd-mmm-yy, dd-mmm-yyyy, dd mmm yy, dd mmm yyyy
and stores the result of the parse in the elt.  If the parse is
successful, T is returned, else NIL.


unsigned long mail_longdate (MESSAGECACHE *elt);
	elt	message cache element containing date.

      This function accepts a message cache element that contains date
information, and returns the number of days since the base time of the
imap-4 toolkit.  At present, this is the same as the Unix time() value
for that date/time, and hence can be used for functions such as utime().

			  Utility Functions

void mail_debug (MAILSTREAM *stream);
	stream	stream to debug

      This function enables telemetry logging for this stream.  All
telemetry is passed to the application via the mm_dlog() facility.


void mail_nodebug (MAILSTREAM *stream);
	stream	stream to disable debugging

     This function disables telemetry logging for this stream.


long mail_sequence (MAILSTREAM *stream,char *sequence);
	stream	stream to set the sequence bits
	sequence IMAP-format message set string

     This function parses the given sequence string for message
numbers, sets the sequence bit in the stream's message cache element
of all messages in the sequence (and turns it off in all other message
cache elements).  If the parse is successful, T is returned, else NIL.


long mail_uid_sequence (MAILSTREAM *stream,char *sequence);
	stream	stream to set the sequence bits
	sequence IMAP-format message set string

     This function parses the given sequence string for unique
identifiers, sets the sequence bit in the stream's message cache
element of all messages in the sequence (and turns it off in all other
message cache elements).  If the parse is successful, T is returned,
else NIL.


long mail_parse_flags (MAILSTREAM *stream,char *flag,unsigned long *uf);
	stream	stream (used to get user flags)
	flag	IMAP-format flag string to parse
	uf	returned location of user flags

     The function parses the given flag string, and returns the system
flags as its return value and the user flags in the location pointed
to by the uf argument.  If there is an error in parse, a log message
is issued via mm_log() and this function returns NIL.


unsigned long mail_filter (char *text,unsigned long len,STRINGLIST *lines,
			   long flags);
	text	RFC 822 text to filter
	len	length in octets in the text argument
	lines	string list of header file names to filter
	flags	option flags

     This function supports the header lines filtering function of
mail_fetchheader_full().  The lines argument contains a list of header
field names to use in subsetting the header text.  Only those lines
which have that header field name are returned, unless FT_NOT is set
in which case only those lines which do not have that header field
name are returned.

     The options for mail_filter() are a bit mask with one or more of
the following:
	FT_NOT		The returned header lines are those that are
			 not in the lines argument


long mail_search_msg (MAILSTREAM *stream,unsigned long msgno,char *charset,
		      SEARCHPGM *pgm);
	stream	stream to search
	msgno	message number of message to inspect
	charset	character set of search strings
	pgm	search program to test

     This function implements mail_search_full() locally in cases when
it is not done by a server (e.g. local mail files, NNTP/POP).  It
inspects the given message on that stream to see if it matches the
criteria or not.  If it matches, T is returned, else NIL.


SEARCHPGM *mail_criteria (char *criteria);
	criteria IMAP2-format search criteria string

     This function accepts an IMAP2-format search criteria string and
parses it.  If the parse is successful, it returns a search program
suitable for use in mail_search_full().
	WARNING: This function does not accept IMAP4 search criteria.
	The source string must be writeable (this restriction was also
	in the old IMAP2 c-client).

	   Data Structure Instantiation/Destruction functions

     These functions are used to obtain structures from free storage and
to release them.

ENVELOPE *mail_newenvelope (void);
ADDRESS *mail_newaddr (void);
BODY *mail_newbody (void);
BODY *mail_initbody (BODY *body);
PARAMETER *mail_newbody_parameter (void);
PART *mail_newbody_part (void);
STRINGLIST *mail_newstringlist (void);
SEARCHPGM *mail_newsearchpgm (void);
SEARCHHEADER *mail_newsearchheader (char *line);
SEARCHSET *mail_newsearchset (void);
SEARCHOR *mail_newsearchor (void);
SEARCHPGMLIST *mail_newsearchpgmlist (void);
SORTPGM *mail_newsortpgm (void);

     These functions, all named mail_new...(), create a new structure of
the given type and initialize all of its elements to zero or empty.

void mail_free_body (BODY **body);
void mail_free_body_parameter (PARAMETER **parameter);
void mail_free_body_part (PART **part);
void mail_free_cache (MAILSTREAM *stream);
void mail_free_elt (MESSAGECACHE **elt);
void mail_free_lelt (LONGCACHE **lelt);
void mail_free_envelope (ENVELOPE **env);
void mail_free_address (ADDRESS **address);
void mail_free_stringlist (STRINGLIST **string);
void mail_free_searchpgm (SEARCHPGM **pgm);
void mail_free_searchheader (SEARCHHEADER **hdr);
void mail_free_searchset (SEARCHSET **set);
void mail_free_searchor (SEARCHOR **orl);
void mail_free_searchpgmlist (SEARCHPGMLIST **pgl);
void mail_free_sortpgm (SORTPGM **pgm);

     These functions, all named mail_free_...(), take a pointer to a
structure pointer, free all contained strings and structures within the
structure, and finally free the structure itself and set its pointer to
NIL.  For example, mail_free_envelope() frees all the ADDRESS structures
contained in the envelope.

     Normally, mail_free_elt() and mail_free_lelt() are used only if the
main program has a private pointer to cache elements.  If so, it is
expected to increment the cache element's lockcount when it makes a
private pointer, and to call this function when it is finished with it.

		       Authentication Functions

char *mail_auth (char *mechanism,authresponse_t resp,int argc,char *argv[]);
	mechanism authentication mechanism name
	resp	callback function for providing responses
	argc	main() function argc value
	argv	main() function argv value

     This server function searches the list of authenticators that was
established by auth_link() for an authenticator with the given name.  If
an authenticator is found, authentication is initialized.  The function
pointed to by resp is called as the authenticator requires responses.


AUTHENTICATOR *mail_lookup_auth (unsigned int i);
	i	position in authenticator list

     This function returns the nth authenticator in the list, where n is
the value of it.


unsigned int mail_lookup_auth_name (char *mechanism);
	mechanism authentication mechanism name

     This function searches the list of authenticators for an
authenticator with the given name, and returns its position in the
authenticator list.


     The functions below are provided by c-client client drivers or by
servers to support the protocol-dependent parts of authentication.

typedef void *(*authchallenge_t) (void *stream,unsigned long *len);
	stream	stream to read challenge
	len	pointer to returned length in octets

     This driver function is called by an authenticator to read a
challenge from the given protocol stream in a protocol-dependent way.
It returns that challenge in binary and its length in octets to the
authenticator.


typedef long (*authrespond_t) (void *stream,char *s,unsigned long size);
	stream	stream to send response
	s	response string
	size	length of response string in octets

     This driver function is called by an authenticator to send a
challenge response to the given stream in a protocol-dependent way.
It returns T if successful, NIL if failure.


typedef char *(*authresponse_t) (void *challenge,unsigned long clen,
				 unsigned long *rlen);
	challenge challenge string
	clen	length of challenge string in octets
	rlen	pointer to returned length of response string

     This server function is called with a challenge string of clen
octets.  It sends, according to whatever protocol (IMAP, POP, etc.) it
uses, and returns the received response and response length in octets.


typedef long (*authclient_t) (authchallenge_t challenger,
			      authrespond_t responder,NETMBX *mb,void *s,
			      unsigned long trial);
	challenger pointer to protocol-dependent challenge reader function
	responder pointer to protocol-dependent response sender function
	mb	NETMBX struct of the mailbox desired to open
	s	stream for protocol-dependent routines to use
	trial	number of authentication attempts remaining

     This client authenticator function negotiates reading challenges
and sending responses for a particular authenticator (Kerberos, etc.)
over the protocol, and returns T if authenticated or NIL if failed.


typedef char *(*authserver_t) (authresponse_t responder,int argc,char *argv[]);
	responder pointer to protocol-dependent responder function
	argc	main() function argc value
	argv	main() function argv value

    This server authenticator function negotiates sending challenges and
reading responses for a particular authenticator (Kerberos, etc.), and
returns either the authenticated user name or NIL if authentication
failed.

		       Network Access Functions

     These functions provide a layer of indirection between the TCP
routines and upper level routines.  This makes it possible to insert
additional code (e.g. privacy or checksum handling).

NETSTREAM *net_open (char *host,char *service,unsigned long port);
	host	host name
	service	contact service name
	port	contact port number

     This function opens a TCP connection to the given host and service
or port.


NETSTREAM *net_aopen (NETMBX *mb,char *service,char *usrbuf);
	NETMBX	parsed mailbox specification
	service	stream to open (at present, only /etc/rimapd is used)
	usrbuf	buffer to return login user name

     This function attempts to open a preauthenticated connection to the
given mailbox and service.  It will return the login user name of the
preauthenticated connection, as well as an open network stream, if
successful.


char *net_getline (NETSTREAM *stream);
	stream	network stream to read

     This routine reads a text line from the stream.  It calls
stream->dtb->getline, which normally points to tcp_getline() but can be
set to some other function.


long net_getbuffer (void *stream,unsigned long size,char *buffer);
	stream	network stream to read
	size	length of data in octets
	buffer	buffer of at least size octets

     This routine reads data from the stream.  It calls
stream->dtb->getbuffer, which normally points to tcp_getbuffer() but can
be set to some other function.


long net_soutr (NETSTREAM *stream,char *string);
	stream	network stream to write
	string	null-terminated string to output

     This routine writes a null-terminated string to the stream.  It
calls stream->dtb->soutr, which normally points to tcp_soutr() but can
be set to some other function.


long net_sout (NETSTREAM *stream,char *string,unsigned long size);
	stream	network stream to write
	string	string to output
	size	length of string in octets

     This routine writes a string of length size to the stream.  It
calls stream->dtb->sout, which normally points to tcp_sout() but can be
set to some other function.


void net_close (NETSTREAM *stream);
	stream	stream to close

     This routine closes the stream.  It calls stream->dtb->close, which
normally points to tcp_close() but can point to some other function.


char *net_host (NETSTREAM *stream);
	stream	stream to inspect

     This routine returns the remote host name of the stream.  It calls
stream->dtb->host, which normally points to tcp_host() but can point
to some other function.


unsigned long net_port (NETSTREAM *stream);
	stream	stream to inspect

     This routine returns the remote port number of the stream.  It calls
stream->dtb->port, which normally points to tcp_port() but can point
to some other function.


char *net_localhost (NETSTREAM *stream);
	stream	stream to inspect

     This routine returns the local host name of the stream.  It calls
stream->dtb->localhost, which normally points to tcp_localhost() but can
point to some other function.

		  Subscription Management Functions

long sm_subscribe (char *mailbox);
	mailbox	mailbox name to subscribe

     This function adds the given mailbox name to the local subscription
list, and returns T if successful, NIL if failure.


long sm_unsubscribe (char *mailbox);
	mailbox	mailbox name to unsubscribe

     This function removes the given mailbox name from the local
subscription list, and returns T if successful, NIL if failure.

char *sm_read (void **sdb);
	sdb	data to use in subsequent calls, or NIL if first call

     This function returns the local subscription list as null
terminated strings.  Each call returns the next element in the list.
The first call should be with sdb pointing to a NIL pointer; this will
be filled in for subsequent calls.  At the last call, NIL will be
returned.

		    Miscellaneous Utility Functions

char *ucase (char *string);
	string	string to convert

     This function converts each lowercase character of the specified
string to uppercase and returns the string.


char *lcase (char *string);
	string	string to convert

     This function converts each uppercase character of the specified
string to lowercase and returns the string.


char *cpystr (char *string);
	string	string to copy

 This function makes a copy of the string from free storage and returns
the copy.


long find_rightmost_bit (long *valptr);
	valptr	pointer to value to search

      This function returns -1 if the 32-bit value pointed to by valptr
is non-zero, otherwise it returns the bit number (0 = LSB, 31 = MSB) of
the right-most bit in that value.  This is used to convert from the bits
in the cache's userflags item to an index into the stream's userFlags
array of flag texts.


long min (long i,long j);
	i	first argument
	j	second argument

      This function returns the minimum of the two integers.

long max (long i,long j);
	i	first argument
	j	second argument

     This function returns the maximum of the two integers.

long search (char *s,long c,char *pat,long patc);
	s	string to search
	c	size of string
	pat	pattern to search in string
	patc	size of pattern

      This function does a fast case-independent search for the given
pattern in pat (length patc) in base string s, and returns T if the
pattern is found in the string.


long pmatch (char *s,char *pat,delim);
long pmatch_full (char *s,char *pat,delim);
	s	string to match
	pat	wildcard (* and %) to match in pattern
	delim	hierarchy delimiter

      This function returns T if the given wildcard pattern matches the
string in s with hierarchy delimiter delim.  Otherwise NIL is returned.


long dmatch (char *s,char *pat,char delim);
	s	string to match
	pat	wildcard (* and %) to match in pattern
	delim	hierarchy delimiter

     This function returns T if the given wildcard pattern matches the
directory.  If not, then none of the elements in the directory are
considered for recursive checking with pmatch_full().

			     SMTP Functions

SMTPSTREAM *smtp_open (char **hostlist,long debug);
	hostlist vector of SMTP server host names to try
	debug	non-zero if want protocol telemetry debugging

      This function opens an SMTP connection to a one of the hosts in the
host list and if successful returns a stream suitable for use by the
other SMTP functions.  The hosts are tried in order until a connection is
successfully opened.  If debug is non-NIL, protocol telemetry is logged
via mm_dlog().  NIL is returned if this function fails to open a
connection to any of the hosts in the list.

void smtp_close (SMTPSTREAM *stream);
	stream	stream to close

     This function closes the SMTP stream and frees all resources
associated with it that it may have created.

long smtp_mail (SMTPSTREAM *stream,char *type,ENVELOPE *msg,BODY *body);
	stream	stream to transmit mail
	type	mail type (MAIL, SEND, SAML, SOML)
	msg	message envelope
	body	message body

      This function negotiates an SMTP transaction of the specified type
(one of "MAIL", "SEND", "SAML", or "SOML") to deliver the specified
message.  This function returns T if success or NIL if there is any
failure.  The text reason for the failure is in stream->reply item; if
it is associated with a recipient it is also in that address'
address->error item.


void smtp_debug (SMTPSTREAM *stream);
	stream	stream to enable debugging telemetry

      This function enables SMTP protocol telemetry logging for this
stream.  All SMTP protocol operations are passed to the application via
the mm_dlog() facility.


void smtp_nodebug (SMTPSTREAM *stream);
	stream	stream to disable debugging telemetry

      This function disables SMTP protocol telemetry logging for this
stream.


typedef void (*smtpverbose_t) (char *buffer);
	buffer	pointer to verbose reply buffer

     This is the argument to the SET_SMTPVERBOSE mail_parmameter() call.
If this function pointer is non-NIL, then if a verbose SMTP response
(with SMTP code less than 100) is received, this function is called with
that response text as its argument.

			     NNTP Functions

NNTPSTREAM *nntp_open (char **hostlist,long debug);
	hostlist vector of NNTP server host names to try
	debug	non-zero if want protocol telemetry debugging

      This function opens an NNTP connection to a one of the hosts in the
host list and if successful returns a stream suitable for use by the
other MTP functions.  The hosts are tried in order until a connection is
successfully opened.  If debug is non-NIL, protocol telemetry is logged
via mm_dlog().  NIL is returned if this function fails to open a
connection to any of the hosts in the list.


void nntp_close (NNTPSTREAM *stream);
	stream	stream to close

     This function closes the NNTP stream and frees all resources
associated with it that it may have created.


long nntp_mail (NNTPSTREAM *stream,ENVELOPE *msg,BODY *body);
	stream	stream to transmit mail
	msg	message envelope
	body	message body

      This function negotiates an NNTP posting transaction to deliver
the specified news message.  This function returns T if success or NIL
if there is any failure.  The text reason for the failure is in
stream->reply item; if it is associated with a recipient it is also in
that address' address->error item.

		      RFC 822 Support Functions

     Although rfc822.c contains several additional functions besides
these, only the functions documented here should be used by
applications.  The other functions are for internal use only.


void rfc822_header (char *header,ENVELOPE *env,BODY *body);
	header	buffer to write RFC 822 header
	env	message ENVELOPE (used to obtain RFC 822 information)
	body	message BODY (used to obtain MIME information)

     This function writes an RFC 822 format header into header based
on the information in the envelope and body.  The header buffer must
be large enough to contain the full text of the resulting header.


void rfc822_write_address (char *dest,ADDRESS *adr);
	dest	buffer to write address list
	adr	RFC 822 ADDRESS list

     This function writes an RFC 822 format address list into dest
based on the information in adr.  The dest buffer must be large enough
to contain the full text of the resulting address list.

void rfc822_parse_msg (ENVELOPE **en,BODY **bdy,char *s,unsigned long i,
		       STRING *b,char *host,char *tmp);
	en	destination pointer where message ENVELOPE will be stored
	bdy	destination pointer where message BODY will be stored
	s	RFC 822 header to parse (character string)
	i	length of RFC 822 header
	b	stringstruct of message body
	host	default host name if an address lacks an @host.
	temp	scratch buffer, must be long enough to hold unwound
		 header lines (a buffer that is i octets long is OK)

     This function parses the RFC 822 header pointed to by s with body
pointed to by string structure b into the specified destination
envelope and body pointers, using host as the default host name and
tmp as a scratch buffer.  New ENVELOPE and BODY structures are
created; when finished with them the application must free them with
mail_free_envelope() and mail_free_body().  Any parsing errors are
noted via the mm_log() mechanism using log type PARSE.


void rfc822_parse_adrlist (ADDRESS **lst,char *string,char *host);
	lst	destination pointer where ADDRESS will be stored
	string	string of addresses to parse
	host	default host name if an address lacks an @host.

     This function parses the address list in the given string into an
address list in lst.  Any addresses missing a host name are have the
host name defaulted from the host argument.  If the destination list
is non-empty it appends the new addresses to the list.  Any parsing
errors are noted via the mm_log() mechanism using log type PARSE.

long rfc822_output (char *t,ENVELOPE *env,BODY *body,soutr_t f,void *s,
		    long ok8bit);
	t	scratch buffer, large enough to hold message header
	env	message ENVELOPE
	body	message BODY
	f	I/O function to write to
	s	stream for I/O function f
	ok8bit	non-zero if OK to output 8-bit data

     This function writes the message described with the given
envelope and body.  Any body part contents of type ENCBINARY is
converted to ENCBASE64 before sending.  If ok8bit is NIL, any message
data of type ENC8BIT is converted to ENCQUOTEDPRINTABLE before
sending; if ok8bit is non-NIL then ENC8BIT data is sent as-is.  T is
returned if the function succeeds, else NIL is returned.

     The function f is typically net_soutr(), but it can be any
function which matches
  typedef long (*soutr_t) (void *stream,char *string);
where stream holds sufficient information to enable the output routine
to know where to output to, and the string is a null-terminated string
to output.  This function returns either T or NIL, and that value is
passed up to rfc822_output() for its return.


void *rfc822_base64 (char *src,unsigned long srcl,unsigned long *len);
	src	source string
	srcl	size of source string in octets
	len	pointer to where destination string length in octets
		 will be returned

     This function decodes a BASE64 body part given a source string
and its length.  The decoded body part as a sequence of binary octets
is returned, and its length is returned in len.


char *rfc822_qprint (char *src,unsigned long srcl,unsigned long *len);
	src	source string
	srcl	size of source string in octets
	len	pointer to where destination string length in octets
		 will be returned

     This function decodes a QUOTED-PRINTABLE body part given a source
string and its length.  The decoded body part as an 8-bit character
string is returned, and its length is returned in len.

	     Operating System-Dependent Public Interface

     These functions are in OS-dependent code, and are rewritten each
time c-client is ported to a new operating system.


void rfc822_date (char *date);
	date	buffer to write the date, must be large enough

     This function is called to get the current date and time in an
RFC 822 format string into the given buffer.


void *fs_get (size_t size);
	size	number of octets requested

      This function allocates and returns a block of free storage of
the specified size.  Unlike malloc(), there is no failure return; this
function must return with the requested storage.


void fs_resize (void **block,size_t size);
	block	pointer to pointer to block to be resized
	size	new size in octets

     This function resizes the free storage block, updating the
pointer if necessary.  Unlike realloc(), there is no failure return;
this function must return with the requested storage.


void fs_give (void **block);
	block	pointer to pointer to block to free

      This function releases a block of free storage allocated by
fs_get().  It also erases the block pointer, so it isn't necessary to
do this in the application.


void fatal (char *string);
	string	message string

      This function is called when an "impossible" error is detected
and the client wishes to crash.  The string should contain a reason.


char *strcrlfcpy (char **dst,long *dstl,char *src,long srcl);
	dst	pointer to destination string pointer
	dstl	pointer to destination string size
	src	source strin
	srcl	source string size

      This function is called to copy into a destination string dst of
size dstl (resized if necessary), a CRLF newline form string from
local format string src of size srcl.


TCPSTREAM *tcp_open (char *host,long port);
TCPSTREAM *tcp_aopen (char *host,char *service);
char *tcp_getline (TCPSTREAM *stream);
long tcp_getbuffer (TCPSTREAM *stream,long size,char *buffer);
long tcp_soutr (TCPSTREAM *stream,char *string);
void tcp_close (TCPSTREAM *stream);
char *tcp_host (TCPSTREAM *stream);
unsigned long tcp_port (TCPSTREAM *stream);
char *tcp_localhost (TCPSTREAM *stream);

     These functions are TCP-specific versions of the more general
net_xxx() functions.  These should not be called directly by
applications.


char *tcp_clienthost (char *dst);
	dst	destination string buffer

     This function should be called only by a server called by inetd
or similar mechanism which maps standard input to a network socket.
It returns the host name of the other end (e.g. the client of a
server) using the given string buffer, or NIL if it can't get this
information.

			Main Program Callbacks

     All applications which use the c-client must have the following
callbacks to handle events from c-client.  Note that in any callback
which involves a mail stream, the stream is locked and you can not
recursively call c-client from the callback.  This may also be true in
callbacks which do not have a stream; in general, the rule is "do not
call c-client, especially any mail_xxx() function, from a c-client
callback".


void mm_flags (MAILSTREAM *stream,unsigned long number);
	stream	stream where event happened
	number	message number

     This function is called when c-client manipulates the flags for
the given message number.  This alerts the application that it may
need to inspect that message's flags to see if there are any
interesting changes.


void mm_status (MAILSTREAM *stream,char *mailbox,MAILSTATUS *status);
	stream	stream where event happened
	mailbox	mailbox name for this status
	status	MAILSTATUS structure with message status

     This function is called when c-client reports status of a mailbox
(generally as the result of a mail_status() function call).  The
returned MAILSTATUS structure has the following members:

long flags;			validity flags.  These are the same as
				 the SA_xxx option flags in the
				 mail_status() call, and they indicate
				 which of the other members of the
				 MAILSTATUS structure have usable data
				 (i.e. if SA_MESSAGES is not set, do
				 not believe status->messages!!).
unsigned long messages;		number of messages if SA_MESSAGES
unsigned long recent;		number of recent messages if SA_RECENT
unsigned long unseen;		number of unseen messages if SA_UNSEEN
unsigned long uidnext;		next UID to be assigned if SA_UIDNEXT
unsigned long uidvalidity;	UID validity value if SA_UIDVALIDITY


void mm_searched (MAILSTREAM *stream,unsigned long number);
	stream	stream where event happened
	number	message number

     This function is called to notify the main program that this
message number matches a search (generally as the result of a
mail_search_full() function call).


void mm_exists (MAILSTREAM *stream,unsigned long number);
	stream	stream where event happened
	number	message number

     This function is called to notify the main program that there are
this many messages in the mailbox.  It is also used to notify the main
program of new mail, by announcing a higher number than the main
program was previously aware.


void mm_expunged (MAILSTREAM *stream,unsigned long number);
	stream	stream where event happened
	number	message number

     This function is called to notify the main program that this
message number has been expunged from the mail file and that all
subsequent messages are now referenced by a message number one less
than before.  This implicitly decrements the number of messages in the
mailbox.


void mm_list (MAILSTREAM *stream,char delim,char *name,long attrib);
	stream	stream where event happened
	delim	hierarchy delimiter
	name	mailbox name
	attrib	mailbox attributes

     This function is called to notify the main program that this
mailbox name matches a mailbox listing request (generally as the
result of a mail_list() function call).  The hierarchy delimiter is a
character that separates out levels of hierarchy in mailbox names.
The attributes are a bit mask with one of the following:
	LATT_NOINFERIORS
			it is not possible for there to be any
			 hierarchy inferiors to this name (that is,
			 this name followed by the hierarchy delimiter
			 and additional name characters).
	LATT_NOSELECT	this is not a mailbox name, just a hierarchy
			 level, and it may not be opened by mail_open()
	LATT_MARKED	this mailbox may have recent messages
	LATT_UNMARKED	this mailbox does not have any recent messages


void mm_lsub (MAILSTREAM *stream,char delim,char *name,long attrib);
	stream	stream where event happened
	delim	hierarchy delimiter
	name	mailbox name
	attrib	mailbox attributes


     This function is called to notify the main program that this
mailbox name matches a subscribed mailbox listing request (generally
as the result of a mail_lsub() function call).  The hierarchy
delimiter is a character that separates out levels of hierarchy in
mailbox names.  The attributes are a bit mask with one of the
following:
	LATT_NOINFERIORS
			it is not possible for there to be any
			 hierarchy inferiors to this name (that is,
			 this name followed by the hierarchy delimiter
			 and additional name characters).
	LATT_NOSELECT	this is not a mailbox name, just a hierarchy
			 level, and it may not be opened by mail_open()
	LATT_MARKED	this mailbox may have recent messages
	LATT_UNMARKED	this mailbox does not have any recent messages


void mm_notify (MAILSTREAM *stream,char *string,long errflg);
	stream	stream where event happened
	string	message string
	errflg	message error level

     This function is called to deliver a stream-oriented message
event.  This is the mechanism by which any IMAP response codes for any
application (e.g. TRYCREATE) are delivered to the application.
No newline is included in the string, so this function has to output
its own.

     The message error level is one of the following:

	NIL	normal operation.  The text is `babble' that may be
		interesting to the user, e.g. the greeting message
		from a server.

	WARN	A warning event.  This event should be displayed to
		the user.  Examples: a mailbox rewrite failed because
		of disk full, but the previous mailbox contents were
		recovered.

	ERROR	An error event.  This event should be displayed to
		the user, or at least logged someplace.  This type of
		error shouldn't happen, and so should be called to the
		attention of support staff.  Whatever happened has
		probably disrupted the user's work.  Examples: an
		untagged BAD from an IMAP server.


void mm_log (char *string,long errflg);
	string	message string
	errflg	message error level

      This function is called to deliver a log message.  No newline is
included in the string, so this function has to output its own.  In
general, it is intended that these messages are logged someplace, and
possibly shown to the user.

     The message error level is one of the following:

	NIL	normal operation.  The text is `babble' that may be
		interesting to the user, e.g. "Expunged 3 messages".

	PARSE	An RFC 822 parsing error.  Since bogus headers are
		all-too-common in the real world, these can often be
		ignored on the "garbage in, garbage out" princple.
		However, since surprising results can be yielded when
		trying to parse garbage, this message should be logged
		somewhere so it can be figured out what happened.

	WARN	A warning event.  This event should be displayed to
		the user.  It occurs when an error condition has
		happened, but c-client knows what to do to recover.
		Examples: "Can't open read-write, so opening
		read-only", "Empty mailbox", "Login failed, try
		again", "Waiting for mailbox to become unlocked",
		"IMAP protocol error".  Although a user should be
		told about a warning, it's generally not necessary
		to interrupt the flow of her work (e.g. it's alright
		to display the warning in a scrolling window, but
		not necessary to require the user to do anything).

	ERROR	An error event.  This event should be displayed to
		the user, or at least logged someplace.  This is a
		serious error condition occured that aborted the
		requested operation and possibly also aborted the mail
		stream.  This ranges from normal error conditions such
		as "Can't open mailbox", "too many login failures, go
		away" to bizarre conditions such as "Apparent new mail
		appeared in the mailbox that doesn't look like mail,
		program aborting".  Errors must be called to the
		user's attention, and probably should require some
		sort of acknowledgement (e.g. answering a modal panel)
		before the application proceeds.


void mm_dlog (char *string);
	string message string

      This function is called to deliver a debugging telemetry
message.  No newline is included in the string, so this function has
to output its own.  This is called only when debugging is enabled.


void mm_login (NETMBX *mb,char *user,char *pwd,long trial);
	mb	parsed mailbox specification
	user	pointer to where to return user name
	pwd	pointer to where to return password
	trial	number of prior login attempts

      This function is called to get a user name and password for the
given network mailbox.  It stores the user name and password in the
strings pointed to by the appropriate arguments.  The trial argument
is the number of attempts to perform the login and is initially zero
(e.g. for a default username and password login functionality).  It is
incremented for each subsequent trial until the maximum number of
trials are made.


void mm_critical (MAILSTREAM *stream);
	stream	stream where event happened

      This function is called to alert the application that c-client
is about to run some critical code on that stream that may result in a
clobbered mail file if it is interrupted.  It may be desirable to
disable CTRL/C, etc. during this time.


void mm_nocritical (MAILSTREAM *stream);
	stream	stream where event happened

      This function is called to alert the application that c-client
is no longer running critical code on that stream that may result in a
clobbered mail file if it is interrupted.


long mm_diskerror (MAILSTREAM *stream,long errcode,long serious);
	stream	stream where event happened
	errcode	OS error code for disk error
	serious	non-zero if c-client can not undo the operation (and
		 thus must retry to avoid mail file damage)

      This function is called to alert the application that the
c-client has encountered an unrecoverable write error when trying to
update the mail file.  errcode contains the system error code.  If
serious is non-zero, then it is probable that the disk copy of the
mailbox has been damaged.

     The return value from this function is the abort flag; if serious
is zero and the abort flag is non-zero, the operation is aborted.  If
the abort flag is zero or if serious was non-zero, a return from this
function will retry the failing operation.


void mm_fatal (char *string);
	string	message string

      This function is called from the fatal() routine in the
operating system code to notify the main program that it is about to
crash.  The string contains a reason.  At the very minimum, the main
program should do something like
 mm_log (string,ERROR);
and then return.  No newline is included in the string, so this
function has to output its own.

			     Driver interface

     When writing a new driver for the c-client, you must provide a
DRIVER stucture giving a dispatch vector between MAIL and the driver.
The DRIVER dispatch vector is described in mail.h.

char *name;
     Name by which the driver is known to c-client.

unsigned long flags;
     Attribute flags for this driver:
	DR_DISABLE	This driver is currently disabled.
	DR_LOCAL	This driver deals with local mailboxes; if
			 this is off it deals with mailboxes over a
			 network.
	DR_MAIL		This driver supports e-mail messages.
	DR_NEWS		This driver supports netnews messages
	DR_READONLY	This driver only allows read-only access;
			 mail_setflag(), mail_expunge(), etc. are
			 no-ops.
	DR_NOFAST	This driver does not implement mail_fetchfast()
			 in a fast way (e.g. it may have to fetch the
			 entire message text over a network to
			 calculate sizes).
	DR_NAMESPACE	This driver accepts and uses namespace format
			 names.
	DR_LOWMEM	This driver is designed for systems with very
			 limited amounts of memory (e.g. DOS) and
			 support routines called by this driver should
			 try not to use much memory.

DRIVER *next;
     Pointer to the next driver which this application supports (or NIL if
this is the last driver).  Drivers are lunk together via the mail_link()
function.

DRIVER *driver_valid (char *mailbox);
     This function returns a pointer to the driver's DRIVER dispatch
vector iff this driver accepts the given name as a valid mailbox for this
driver.  Otherwise, it returns the value of the next driver's
driver_valid() or NIL if there is no next driver.  In other words, calling
driver_valid() for the first driver will return the driver dispatch vector
for the driver which supports this type of mailbox.

void *driver_parameters (long function,void *value);
     This function implements mail_parameters() for this driver.

void driver_scan (MAILSTREAM *stream,char *ref,char *pat,char *contents);
     This function implements mail_scan() for this driver.

void driver_list (MAILSTREAM *stream,char *ref,char *pat);
     This function implements mail_list() for this driver.

void driver_lsub (MAILSTREAM *stream,char *ref,char *pat);
     This function implements mail_lsub() for this driver.

long driver_subscribe (MAILSTREAM *stream,char *mailbox);
     This function implements mail_subscribe() for this driver.

long driver_unsubscribe (MAILSTREAM *stream,char *mailbox);
     This function implements mail_unsubscribe() for this driver.

long driver_create (MAILSTREAM *stream,char *mailbox);
     This function implements mail_create() for this driver.

long driver_delete (MAILSTREAM *stream,char *mailbox);
     This function implements mail_delete() for this driver.

long driver_rename (MAILSTREAM *stream,char *old,char *new);
     This function implements mail_rename() for this driver.

long driver_status (MAILSTREAM *stream,char *mailbox,long flags);
     This function implements mail_status() for this driver.

MAILSTREAM *driver_open (MAILSTREAM *stream);
     This function opens the mailbox identified by the given stream.  It
may use the data on the stream and create additional data on stream->local
as necessary.  It should return the given stream unless it failed to open
the mailbox, in which case it should return NIL.

void driver_close (MAILSTREAM *stream,long options);
     This function implements mail_close() for this driver.

void driver_fetchfast (MAILSTREAM *stream,char *sequence,long flags);
     This function implements mail_fetchfast() for this driver.

void driver_fetchflags (MAILSTREAM *stream,char *sequence,long flags);
     This function implements mail_fetchflags() for this driver.

ENVELOPE *driver_fetchstructure (MAILSTREAM *stream,unsigned long msgno,
				 BODY **body,long flags);
     This function implements mail_fetchstructure() for this driver.

char *driver_fetchheader (MAILSTREAM *stream,unsigned long msgno,
			  STRINGLIST *lines,unsigned long *len,long flags);
     This function implements mail_fetchheader() for this driver.

char *driver_fetchtext (MAILSTREAM *stream,unsigned long msgno,
			unsigned long *len,long flags);
     This function implements mail_fetchtext() for this driver.

char *driver_fetchbody (MAILSTREAM *stream,unsigned long msgno,char *section,
			unsigned long *len,long flags);
     This function implements mail_fetchbody() for this driver.

void driver_setflag (MAILSTREAM *stream,char *sequence,char *flag,long flags);
     This function implements mail_setflag() for this driver.

void driver_clearflag (MAILSTREAM *stream,char *sequence,char *flag,
		       long flags);
     This function implements mail_clearflag() for this driver.

void driver_search (MAILSTREAM *stream,char *charset,SEARCHPGM *pgm,
		    long flags);
     This function implements mail_search() for this driver.

unsigned long *driver_sort (MAILSTREAM *stream,char *charset,SEARCHPGM *spg,
			    SORTPGM *pgm,long flags);
     This function implements mail_sort() for this driver.

void *driver_thread (MAILSTREAM *stream,char *seq,long function,long flag);
     This dispatch is reserved for a future threading capability.

long driver_ping (MAILSTREAM *stream);
      This function implements mail_ping() for this driver.

void driver_check (MAILSTREAM *stream);
      This function implements mail_check() for this driver.

void driver_expunge (MAILSTREAM *stream);
      This function implements mail_expunge() for this driver.

long driver_copy (MAILSTREAM *stream,char *sequence,char *mailbox,
		  long options);
      This function implements mail_copy() for this driver.

long driver_append (MAILSTREAM *stream,char *mailbox,char *flags,char *date,
		    STRING *message);
      This function implements mail_append() for this driver.

void driver_gc (MAILSTREAM *stream,long gcflags);
      This function implements mail_gc() for this driver.

			 Driver Support Functions

void mail_searched (MAILSTREAM *stream,unsigned long msgno);
	stream	stream where event happened
	msgno	message number

     This function is called by the driver to notify c-client that this
message number matches a search.  It invokes the main program's
mm_searched() function.

void mail_exists (MAILSTREAM *stream,unsigned long nmsgs);
	stream	stream where event happened
	nmsgs	number of messages

     This function is called by the driver to notify c-client that this
message number exists (i.e. there are this many messages in the mailbox).
It invokes the main program's mm_exists() function.

void mail_recent (MAILSTREAM *stream,unsigned long recent);
	stream	stream where event happened
	recent	number of messages

      This function is called by the driver to notify c-client that this
many messages are "recent" (i.e. arrived in the mailbox since the previous
time the mailbox was opened).

void mail_expunged (MAILSTREAM *stream,unsigned long msgno);
	stream	stream where event happened
	msgno	number of messages

      This function is called by the driver to notify MAIL that this
message number has been expunged from the mail file and that all subsequent
messages are now referenced by a message number one less than before.  It
invokes the main program's mm_expunged() function.

void mail_lock (MAILSTREAM *stream);
	stream	stream where event happened
      This function sets the stream lock.  It is an error to set the stream
lock if the stream is already locked.

      This is mainly used to catch errors due to a callback function
(e.g. mm_exists) inadvertantly recursing back to the MAIL routines and
establishing an infinite recursion.  Normally, drivers will set the lock
prior to calling one of the callback functions above or, more likely, in
the beginning of the driver's non-reentrant "do operation" section.  In the
IMAP4 driver, the stream lock is set when entering imap_send() and cleared
on exit.

void mail_unlock (MAILSTREAM *stream);
	stream	stream where event happened

     This function releases the stream lock.  It is an error to release the
stream lock if the stream is not locked.