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pdfio/doc/pdfio.md
Michael R Sweet 7e9c0afc23
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Introduction

PDFio is a simple C library for reading and writing PDF files. The primary goals of pdfio are:

  • Read and write any version of PDF file
  • Provide access to pages, objects, and streams within a PDF file
  • Support reading and writing of encrypted PDF files
  • Extract or embed useful metadata (author, creator, page information, etc.)
  • "Filter" PDF files, for example to extract a range of pages or to embed fonts that are missing from a PDF
  • Provide access to objects used for each page

PDFio is not concerned with rendering or viewing a PDF file, although a PDF RIP or viewer could be written using it.

PDFio is Copyright © 2021 by Michael R Sweet and is licensed under the Apache License Version 2.0 with an (optional) exception to allow linking against GPL2/LGPL2 software. See the files "LICENSE" and "NOTICE" for more information.

Requirements

PDFio requires the following to build the software:

  • A C99 compiler such as Clang, GCC, or MS Visual C
  • A POSIX-compliant make program
  • ZLIB (https://www.zlib.net) 1.0 or higher

IDE files for Xcode (macOS/iOS) and Visual Studio (Windows) are also provided.

Installing pdfio

PDFio comes with a portable makefile that will work on any POSIX-compliant system with ZLIB installed. To make it, run:

make all

To test it, run:

make test

To install it, run:

make install

If you want a shared library, run:

make all-shared
make install-shared

The default installation location is "/usr/local". Pass the prefix variable to make to install it to another location:

make install prefix=/some/other/directory

The makefile installs the pdfio header to "${prefix}/include", the library to "${prefix}/lib", the pkg-config file to "${prefix}/lib/pkgconfig", the man page to "${prefix}/share/man/man3", and the documentation to "${prefix}/share/doc/pdfio".

The makefile supports the following variables that can be specified in the make command or as environment variables:

  • AR: the library archiver (default "ar")
  • ARFLAGS: options for the library archiver (default "cr")
  • CC: the C compiler (default "cc")
  • CFLAGS: options for the C compiler (default "")
  • CODESIGN_IDENTITY: the identity to use when code signing the shared library on macOS (default "Developer ID")
  • COMMONFLAGS: options for the C compiler and linker (typically architecture and optimization options, default is "-Os -g")
  • CPPFLAGS: options for the C preprocessor (default "")
  • DESTDIR and DSTROOT: specifies a root directory when installing (default is "", specify only one)
  • DSOFLAGS: options for the C compiler when linking the shared library (default "")
  • LDFLAGS: options for the C compiler when linking the test programs (default "")
  • LIBS: library options when linking the test programs (default "-lz")
  • RANLIB: program that generates a table-of-contents in a library (default "ranlib")
  • prefix: specifies the installation directory (default "/usr/local")

Visual Studio Project

The Visual Studio solution ("pdfio.sln") is provided for Windows developers and generates both a static library and DLL.

Xcode Project

There is also an Xcode project ("pdfio.xcodeproj") you can use on macOS which generates a static library that will be installed under "/usr/local" with:

sudo xcodebuild install

You can reproduce this with the makefile using:

sudo make COMMONFLAGS="-Os -mmacosx-version-min=10.14 -arch x86_64 -arch arm64" install

Detecting PDFio

PDFio can be detected using the pkg-config command, for example:

if pkg-config --exists pdfio; then
    ...
fi

In a makefile you can add the necessary compiler and linker options with:

CFLAGS  +=      `pkg-config --cflags pdfio`
LIBS    +=      `pkg-config --libs pdfio`

On Windows, you need to link to the PDFIO1.LIB (DLL) library and include the zlib_native NuGet package dependency. You can also use the published pdfio_native NuGet package.

Header Files

PDFio provides a primary header file that is always used:

#include <pdfio.h>

PDFio also provides PDF content helper functions for producing PDF content that are defined in a separate header file:

#include <pdfio-content.h>

API Overview

PDFio exposes several types:

  • pdfio_file_t: A PDF file (for reading or writing)
  • pdfio_array_t: An array of values
  • pdfio_dict_t: A dictionary of key/value pairs in a PDF file, object, etc.
  • pdfio_obj_t: An object in a PDF file
  • pdfio_stream_t: An object stream

Reading PDF Files

You open an existing PDF file using the pdfioFileOpen function:

pdfio_file_t *pdf = pdfioFileOpen("myinputfile.pdf", error_cb, error_data);

where the three arguments to the function are the filename ("myinputfile.pdf"), an optional error callback function (error_cb), and an optional pointer value for the error callback function (error_data). The error callback is called for both errors and warnings and accepts the pdfio_file_t pointer, a message string, and the callback pointer value, for example:

bool
error_cb(pdfio_file_t *pdf, const char *message, void *data)
{
  (void)data; // This callback does not use the data pointer

  fprintf(stderr, "%s: %s\n", pdfioFileGetName(pdf), message);

  // Return false to treat warnings as errors
  return (false);
}

The default error callback (NULL) does the equivalent of the above.

Each PDF file contains one or more pages. The pdfioFileGetNumPages function returns the number of pages in the file while the pdfioFileGetPage function gets the specified page in the PDF file:

pdfio_file_t *pdf;   // PDF file
size_t       i;      // Looping var
size_t       count;  // Number of pages
pdfio_obj_t  *page;  // Current page

// Iterate the pages in the PDF file
for (i = 0, count = pdfioFileGetNumPages(pdf); i < count; i ++)
{
  page = pdfioFileGetPage(pdf, i);
  // do something with page
}

Each page is represented by a "page tree" object (what pdfioFileGetPage returns) that specifies information about the page and one or more "content" objects that contain the images, fonts, text, and graphics that appear on the page.

The pdfioFileClose function closes a PDF file and frees all memory that was used for it:

pdfioFileClose(pdf);

Writing PDF Files

You create a new PDF file using the pdfioFileCreate function:

pdfio_rect_t media_box = { 0.0, 0.0, 612.0, 792.0 };  // US Letter
pdfio_rect_t crop_box = { 36.0, 36.0, 576.0, 756.0 }; // w/0.5" margins

pdfio_file_t *pdf = pdfioFileCreate("myoutputfile.pdf", "2.0", &media_box, &crop_box, error_cb, error_data);

where the six arguments to the function are the filename ("myoutputfile.pdf"), PDF version ("2.0"), media box (media_box), crop box (crop_box), an optional error callback function (error_cb), and an optional pointer value for the error callback function (error_data). The units for the media and crop boxes are points (1/72nd of an inch).

Alternately you can stream a PDF file using the pdfioFileCreateOutput function:

pdfio_rect_t media_box = { 0.0, 0.0, 612.0, 792.0 };  // US Letter
pdfio_rect_t crop_box = { 36.0, 36.0, 576.0, 756.0 }; // w/0.5" margins

pdfio_file_t *pdf = pdfioFileCreateOutput(output_cb, output_ctx, "2.0", &media_box, &crop_box, error_cb, error_data);

Once the file is created, use the pdfioFileCreateObj, pdfioFileCreatePage, and pdfioPageCopy functions to create objects and pages in the file.

Finally, the pdfioFileClose function writes the PDF cross-reference and "trailer" information, closes the file, and frees all memory that was used for it.

PDF Objects

PDF objects are identified using two numbers - the object number (1 to N) and the object generation (0 to 65535) that specifies a particular version of an object. An object's numbers are returned by the pdfioObjGetNumber and pdfioObjGetGeneration functions. You can find a numbered object using the pdfioFileFindObj function.

Objects contain values (typically dictionaries) and usually an associated data stream containing images, fonts, ICC profiles, and page content. PDFio provides several accessor functions to get the value(s) associated with an object:

PDF Streams

Some PDF objects have an associated data stream, such as for pages, images, ICC color profiles, and fonts. You access the stream for an existing object using the pdfioObjOpenStream function:

pdfio_file_t *pdf = pdfioFileOpen(...);
pdfio_obj_t *obj = pdfioFileFindObj(pdf, number);
pdfio_stream_t *st = pdfioObjOpenStream(obj, true);

The first argument is the object pointer. The second argument is a boolean value that specifies whether you want to decode (typically decompress) the stream data or return it as-is.

Once you have the stream open, you can use one of several functions to read from it:

When you are done reading from the stream, call the pdfioStreamClose function:

pdfioStreamClose(st);

To create a stream for a new object, call the pdfioObjCreateStream function:

pdfio_file_t *pdf = pdfioFileCreate(...);
pdfio_obj_t *pdfioFileCreateObj(pdf, ...);
pdfio_stream_t *pdfioObjCreateStream(obj, PDFIO_FILTER_FLATE);

The first argument is the newly created object. The second argument is either PDFIO_FILTER_NONE to specify that any encoding is done by your program or PDFIO_FILTER_FLATE to specify that PDFio should Flate compress the stream.

Once you have created the stream, use any of the following functions to write to the stream:

The PDF content helper functions provide additional functions for writing specific PDF page stream commands.

When you are done writing the stream, call pdfioStreamCLose to close both the stream and the object.

PDF Content Helper Functions

PDFio includes many helper functions for embedding or writing specific kinds of content to a PDF file. These functions can be roughly grouped into ??? categories:

Color Space Functions

PDF color spaces are specified using well-known names like "DeviceCMYK", "DeviceGray", and "DeviceRGB" or using arrays that define so-called calibrated color spaces. PDFio provides several functions for embedding ICC profiles and creating color space arrays:

You can embed an ICC color profile using the pdfioFileCreateICCObjFromFile function:

pdfio_file_t *pdf = pdfioFileCreate(...);
pdfio_obj_t *icc = pdfioFileCreateICCObjFromFile(pdf, "filename.icc");

where the first argument is the PDF file and the second argument is the filename of the ICC color profile.

PDFio also includes predefined constants for creating a few standard color spaces:

pdfio_file_t *pdf = pdfioFileCreate(...);

// Create an AdobeRGB color array
pdfio_array_t *adobe_rgb = pdfioArrayCreateColorFromStandard(pdf, 3, PDFIO_CS_ADOBE);

// Create an Display P3 color array
pdfio_array_t *display_p3 = pdfioArrayCreateColorFromStandard(pdf, 3, PDFIO_CS_P3_D65);

// Create an sRGB color array
pdfio_array_t *srgb = pdfioArrayCreateColorFromStandard(pdf, 3, PDFIO_CS_SRGB);

Font Object Functions

PDF supports many kinds of fonts, including PostScript Type1, PDF Type3, TrueType/OpenType, and CID. PDFio provides two functions for creating font objects. The first is pdfioFileCreateFontObjFromBase which creates a font object for one of the base PDF fonts:

  • "Courier"
  • "Courier-Bold"
  • "Courier-BoldItalic"
  • "Courier-Italic"
  • "Helvetica"
  • "Helvetica-Bold"
  • "Helvetica-BoldOblique"
  • "Helvetica-Oblique"
  • "Symbol"
  • "Times-Bold"
  • "Times-BoldItalic"
  • "Times-Italic"
  • "Times-Roman"
  • "ZapfDingbats"

PDFio always uses the Windows CP1252 subset of Unicode for these fonts.

The second function is pdfioFileCreateFontObjFromFile which creates a font object from a TrueType/OpenType font file, for example:

pdfio_file_t *pdf = pdfioFileCreate(...);
pdfio_obj_t *arial = pdfioFileCreateFontObjFromFile(pdf, "OpenSans-Regular.ttf", false);

will embed an OpenSans Regular TrueType font using the Windows CP1252 subset of Unicode. Pass true for the third argument to embed it as a Unicode CID font instead, for example:

pdfio_file_t *pdf = pdfioFileCreate(...);
pdfio_obj_t *arial = pdfioFileCreateFontObjFromFile(pdf, "NotoSansJP-Regular.otf", true);

will embed the NotoSansJP Regular OpenType font with full support for Unicode.

Note: Not all fonts support Unicode.

Image Object Functions

PDF supports images with many different color spaces and bit depths with optional transparency. PDFio provides two helper functions for creating image objects that can be referenced in page streams. The first function is pdfioFileCreateImageObjFromData which creates an image object from data in memory, for example:

pdfio_file_t *pdf = pdfioFileCreate(...);
unsigned char data[1024 * 1024 * 4]; // 1024x1024 RGBA image data
pdfio_obj_t *img = pdfioFileCreateImageObjFromData(pdf, data, /*width*/1024, /*height*/1024, /*num_colors*/3, /*color_data*/NULL, /*alpha*/true, /*interpolate*/false);

will create an object for a 1024x1024 RGBA image in memory, using the default color space for 3 colors ("DeviceRGB"). We can use one of the color space functions to use a specific color space for this image, for example:

pdfio_file_t *pdf = pdfioFileCreate(...);

// Create an AdobeRGB color array
pdfio_array_t *adobe_rgb = pdfioArrayCreateColorFromMatrix(pdf, 3, pdfioAdobeRGBGamma, pdfioAdobeRGBMatrix, pdfioAdobeRGBWhitePoint);

// Create a 1024x1024 RGBA image using AdobeRGB
unsigned char data[1024 * 1024 * 4]; // 1024x1024 RGBA image data
pdfio_obj_t *img = pdfioFileCreateImageObjFromData(pdf, data, /*width*/1024, /*height*/1024, /*num_colors*/3, /*color_data*/adobe_rgb, /*alpha*/true, /*interpolate*/false);

The "interpolate" argument specifies whether the colors in the image should be smoothed/interpolated when scaling. This is most useful for photographs but should be false for screenshot and barcode images.

If you have a JPEG or PNG file, use the pdfioFileCreateImageObjFromFile function to copy the image into a PDF image object, for example:

pdfio_file_t *pdf = pdfioFileCreate(...);
pdfio_obj_t *img = pdfioFileCreateImageObjFromFile(pdf, "myphoto.jpg", /*interpolate*/true);

Page Dictionary Functions

PDF pages each have an associated dictionary to specify the images, fonts, and color spaces used by the page. PDFio provides functions to add these resources to the dictionary:

Page Stream Functions

PDF page streams contain textual commands for drawing on the page. PDFio provides many functions for writing these commands with the correct format and escaping, as needed: