Developing for CUPS

Please see the Contributing to CUPS file for information on contributing to the CUPS project.

How To Contact The Developers

The CUPS mailing lists are the primary means of asking questions and informally discussing issues and feature requests with the CUPS developers and other experienced CUPS users and developers. The “cups” mailing list is intended for CUPS usage questions and new software announcements while the “cups-devel” mailing list provides a forum for CUPS developers and monitoring new bugs.


CUPS interfaces, including the C APIs and command-line arguments, environment variables, configuration files, and output format, are stable across patch versions and are generally backwards-compatible with interfaces used in prior major and minor versions. However, program interfaces such as those used by the scheduler to run filter, port monitor, and backend processes for job processing should only be considered stable from the point of view of a filter, port monitor, or backend. Software that simulates the scheduler in order to run those programs outside of CUPS must necessarily be updated when the corresponding interface is changed in a subsequent CUPS release, otherwise undefined behavior can occur.

CUPS C APIs starting with an underscore (_) are considered to be private to CUPS and are not subject to the normal guarantees of stability between CUPS releases and must never be used in non-CUPS source code. Similarly, configuration and state files written by CUPS are considered private if a corresponding man page is not provided with the CUPS release. Never rely on undocumented files or formats when developing software for CUPS. Always use a published C API to access data stored in a file to avoid compatibility problems in the future.

Build System

The CUPS build system uses GNU autoconf to tailor the library to the local operating system. Project files for the current release of Microsoft Visual Studio are also provided for Microsoft Windows®. To improve portability, makefiles must not make use of features unique to GNU make. See the MAKEFILE GUIDELINES section for a description of the allowed make features and makefile guidelines.

Additional GNU build programs such as GNU automake and GNU libtool must not be used. GNU automake produces non-portable makefiles which depend on GNU- specific extensions, and GNU libtool is not portable or reliable enough for CUPS.

Version Numbering

CUPS uses a three-part version number separated by periods to represent the major, minor, and patch release numbers. Major release numbers indicate large design changes or backwards-incompatible changes to the CUPS API or CUPS Imaging API. Minor release numbers indicate new features and other smaller changes which are backwards-compatible with previous CUPS releases. Patch numbers indicate bug fixes to the previous feature or patch release. This version numbering scheme is consistent with the Semantic Versioning specification.


When we talk about compatibility, we are talking about binary compatibility for public APIs and output format compatibility for program interfaces. Changes to configuration file formats or the default behavior of programs are not generally considered incompatible as the upgrade process can normally address such changes gracefully.

Production releases use the plain version numbers:


The first production release in a MAJOR.MINOR series (MAJOR.MINOR.0) is called a feature release. Feature releases are the only releases that may contain new features. Subsequent production releases in a MAJOR.MINOR series may only contain bug fixes.

Beta-test releases are identified by appending the letter B to the major and minor version numbers followed by the beta release number:


Release candidates are identified by appending the letters RC to the major and minor version numbers followed by the release candidate number:


Coding Guidelines

Contributed source code must follow the guidelines below. While the examples are for C and C++ source files, source code for other languages should conform to the same guidelines as allowed by the language.

Source code comments provide the reference portion of the CUPS Programming Manual, which is generated using the codedoc software.

Source Files

All source files names must be 16 characters or less in length to ensure compatibility with older UNIX filesystems. Source files containing functions have an extension of “.c” for C and “.cxx” for C++ source files. All other “include” files have an extension of “.h”. Tabs are set to 8 characters or columns.


The “.cxx” extension is used because it is the only common C++ extension between Linux, macOS, UNIX, and Windows.

The top of each source file contains a header giving the purpose or nature of the source file and the copyright and licensing notice:

 * Description of file contents.
 * Copyright 2017 by Apple Inc.
 * Licensed under Apache License v2.0.  See the file "LICENSE" for more
 * information.

Header Files

All public header files must include the “versioning.h” header file, or a header that does so. Function declarations are then “decorated” with the correct _CUPS_API_major_minor macro to define its availability based on the build environment, for example:

extern int cupsDoThis(int foo, int bar) _CUPS_API_2_2;

Private API header files must be named with the suffix “-private”, for example the “cups.h” header file defines all of the public CUPS APIs while the “cups-private.h” header file defines all of the private CUPS APIs as well. Typically a private API header file will include the corresponding public API header file.


All source code utilizes block comments within functions to describe the operations being performed by a group of statements; avoid putting a comment per line unless absolutely necessary, and then consider refactoring the code so that it is not necessary. C source files use the block comment format (“/* comment */”) since many vendor C compilers still do not support C99/C++ comments (“// comment”):

 * Clear the state array before we begin...

 for (i = 0; i < (sizeof(array) / sizeof(sizeof(array[0])); i ++)
   array[i] = CUPS_STATE_IDLE;

 * Wait for state changes on another thread...

   for (i = 0; i < (sizeof(array) / sizeof(sizeof(array[0])); i ++)
     if (array[i] != CUPS_STATE_IDLE)

   if (i == (sizeof(array) / sizeof(array[0])))
 } while (i == (sizeof(array) / sizeof(array[0])));


All code blocks enclosed by brackets begin with the opening brace on a new line. The code then follows starting on a new line after the brace and is indented 2 spaces. The closing brace is then placed on a new line following the code at the original indentation:

  int i; /* Looping var */

  * Process foobar values from 0 to 999...

  for (i = 0; i < 1000; i ++)

Single-line statements following “do”, “else”, “for”, “if”, and “while” are indented 2 spaces as well. Blocks of code in a “switch” block are indented 4 spaces after each “case” and “default” case:

switch (array[i])

  default :


A space follows each reserved word such as if, while, etc. Spaces are not inserted between a function name and the arguments in parenthesis.

Return Values

Parenthesis surround values returned from a function:



Functions with a global scope have a lowercase prefix followed by capitalized words, e.g., cupsDoThis, cupsDoThat, cupsDoSomethingElse, etc. Private global functions begin with a leading underscore, e.g., _cupsDoThis, _cupsDoThat, etc.

Functions with a local scope are declared static with lowercase names and underscores between words, e.g., do_this, do_that, do_something_else, etc.

Each function begins with a comment header describing what the function does, the possible input limits (if any), the possible output values (if any), and any special information needed:

 * 'do_this()' - Compute y = this(x).
 * Notes: none.

static float       /* O - Inverse power value, 0.0 <= y <= 1.1 */
do_this(float x)   /* I - Power value (0.0 <= x <= 1.1) */
  return (y);

Return/output values are indicated using an “O” prefix, input values are indicated using the “I” prefix, and values that are both input and output use the “IO” prefix for the corresponding in-line comment.

The codedoc documentation generator also understands the following special text in the function description comment:

@deprecated@         - Marks the function as deprecated: not recommended
                       for new development and scheduled for removal.
@link name@          - Provides a hyperlink to the corresponding function
                       or type definition.
@since CUPS version@ - Marks the function as new in the specified version
                       of CUPS.
@private@            - Marks the function as private so it will not be
                       included in the documentation.


Variables with a global scope are capitalized, e.g., ThisVariable, ThatVariable, ThisStateVariable, etc. Globals in CUPS libraries are either part of the per-thread global values managed by the _cupsGlobals function or are suitably protected for concurrent access. Global variables should be replaced by function arguments whenever possible.

Variables with a local scope are lowercase with underscores between words, e.g., this_variable, that_variable, etc. Any “local global” variables shared by functions within a source file are declared static. As for global variables, local static variables are suitably protected for concurrent access.

Each variable is declared on a separate line and is immediately followed by a comment block describing the variable:

int         ThisVariable;    /* The current state of this */
static int  that_variable;   /* The current state of that */


All type names are lowercase with underscores between words and _t appended to the end of the name, e.g., cups_this_type_t, cups_that_type_t, etc. Type names start with a prefix, typically cups or the name of the program, to avoid conflicts with system types. Private type names start with an underscore, e.g., _cups_this_t, _cups_that_t, etc.

Each type has a comment block immediately after the typedef:

typedef int cups_this_type_t;  /* This type is for CUPS foobar options. */


All structure names are lowercase with underscores between words and _s appended to the end of the name, e.g., cups_this_s, cups_that_s, etc. Structure names start with a prefix, typically cups or the name of the program, to avoid conflicts with system types. Private structure names start with an underscore, e.g., _cups_this_s, _cups_that_s, etc.

Each structure has a comment block immediately after the struct and each member is documented similar to the variable naming policy above:

struct cups_this_struct_s  /* This structure is for CUPS foobar options. */
  int this_member;         /* Current state for this */
  int that_member;         /* Current state for that */


All constant names are uppercase with underscores between words, e.g., CUPS_THIS_CONSTANT, CUPS_THAT_CONSTANT, etc. Constants begin with an uppercase prefix, typically CUPS_ or the program or type name. Private constants start with an underscore, e.g., _CUPS_THIS_CONSTANT, _CUPS_THAT_CONSTANT, etc.

Typed enumerations should be used whenever possible to allow for type checking by the compiler.

Comment blocks immediately follow each constant:

typedef enum cups_tray_e  /* Tray enumerations */
  CUPS_TRAY_THIS,         /* This tray */
  CUPS_TRAY_THAT          /* That tray */
} cups_tray_t;

Makefile Guidelines

The following is a guide to the makefile-based build system used by CUPS. These standards have been developed over the years to allow CUPS to be built on as many systems and environments as possible.

General Organization

The CUPS source code is organized functionally into a top-level makefile, include file, and subdirectories each with their own makefile and dependencies files. The “.in” files are template files for the autoconf software and are used to generate a static version of the corresponding file.

Makefile Documentation

Each makefile starts with the standard CUPS header containing the description of the file, and CUPS copyright and license notice:

# Makefile for ...
# Copyright 2017 by Apple Inc.
# Licensed under Apache License v2.0.  See the file "LICENSE" for more
# information.

Portable Makefile Construction

CUPS uses a common subset of make program syntax to ensure that the software can be compiled “out of the box” on as many systems as possible. The following is a list of assumptions we follow when constructing makefiles:

  • Targets; we assume that the make program supports the notion of simple targets of the form “name:” that perform tab-indented commands that follow the target, e.g.:

    TAB target commands
  • Dependencies; we assume that the make program supports recursive dependencies on targets, e.g.:

    target: foo bar
    TAB target commands
    foo: bla
    TAB foo commands
    TAB bar commands
    TAB bla commands
  • Variable Definition; we assume that the make program supports variable definition on the command-line or in the makefile using the following form:

  • Variable Substitution; we assume that the make program supports variable substitution using the following forms:

    - `$(name)`; substitutes the value of "name",
    - `$(`; substitutes the value of "name" with the filename
      extension ".old" changed to ".new",
    - `$(MAKEFLAGS)`; substitutes the command-line options passed to the
      program without the leading hyphen (-),
    - `$$`; substitutes a single $ character,
    - `$<`; substitutes the current source file or dependency, and
    - `$@`; substitutes the current target name.
  • Suffixes; we assume that the make program supports filename suffixes with assumed dependencies, e.g.:

    .SUFFIXES: .c .o
    TAB $(CC) $(CFLAGS) -o $@ -c $<
  • Include Files; we assume that the make program supports the include directive, e.g.:

    include ../Makedefs
    include Dependencies
  • Comments; we assume that comments begin with a # character and proceed to the end of the current line.

  • Line Length; we assume that there is no practical limit to the length of lines.

  • Continuation of long lines; we assume that the \ character may be placed at the end of a line to concatenate two or more lines in a makefile to form a single long line.

  • Shell; we assume a POSIX-compatible shell is present on the build system.

Standard Variables

The following variables are defined in the “Makedefs” file generated by the autoconf software:

  • ALL_CFLAGS; the combined C compiler options,
  • ALL_CXXFLAGS; the combined C++ compiler options,
  • AMANDIR; the administrative man page installation directory (section 8/1m depending on the platform),
  • AR; the library archiver command,
  • ARFLAGS; options for the library archiver command,
  • AWK; the local awk command,
  • BINDIR; the binary installation directory,
  • BUILDROOT; optional installation prefix (defaults to DSTROOT),
  • CC; the C compiler command,
  • CFLAGS; options for the C compiler command,
  • CHMOD; the chmod command,
  • CXX; the C++ compiler command,
  • CXXFLAGS; options for the C++ compiler command,
  • DATADIR; the data file installation directory,
  • DSO; the C shared library building command,
  • DSOXX; the C++ shared library building command,
  • DSOFLAGS; options for the shared library building command,
  • INCLUDEDIR; the public header file installation directory,
  • INSTALL; the install command,
  • INSTALL_BIN; the program installation command,
  • INSTALL_COMPDATA; the compressed data file installation command,
  • INSTALL_CONFIG; the configuration file installation command,
  • INSTALL_DATA; the data file installation command,
  • INSTALL_DIR; the directory installation command,
  • INSTALL_LIB; the library installation command,
  • INSTALL_MAN; the documentation installation command,
  • INSTALL_SCRIPT; the shell script installation command,
  • LD; the linker command,
  • LDFLAGS; options for the linker,
  • LIBDIR; the library installation directory,
  • LIBS; libraries for all programs,
  • LN; the ln command,
  • MAN1EXT; extension for man pages in section 1,
  • MAN3EXT; extension for man pages in section 3,
  • MAN5EXT; extension for man pages in section 5,
  • MAN7EXT; extension for man pages in section 7,
  • MAN8DIR; subdirectory for man pages in section 8,
  • MAN8EXT; extension for man pages in section 8,
  • MANDIR; the man page installation directory,
  • OPTIM; common compiler optimization options,
  • PRIVATEINCLUDE; the private header file installation directory,
  • RM; the rm command,
  • SHELL; the sh (POSIX shell) command,
  • STRIP; the strip command,
  • srcdir; the source directory.

Standard Targets

The following standard targets are defined in each makefile:

  • all; creates all target programs, libraries, and documentation files,
  • clean; removes all target programs libraries, documentation files, and object files,
  • depend; generates automatic dependencies for any C or C++ source files (also see “DEPENDENCIES”),
  • distclean; removes autoconf-generated files in addition to those removed by the “clean” target,
  • install; installs all distribution files in their corresponding locations (also see “INSTALL/UNINSTALL SUPPORT”),
  • install-data; installs all data files in their corresponding locations (also see “INSTALL/UNINSTALL SUPPORT”),
  • install-exec; installs all executable files in their corresponding locations (also see “INSTALL/UNINSTALL SUPPORT”),
  • install-headers; installs all include files in their corresponding locations (also see “INSTALL/UNINSTALL SUPPORT”),
  • install-libs; installs all library files in their corresponding locations (also see “INSTALL/UNINSTALL SUPPORT”), and
  • uninstall; removes all distribution files from their corresponding locations (also see “INSTALL/UNINSTALL SUPPORT”).

Object Files

Object files (the result of compiling a C or C++ source file) have the extension “.o”.


Program files are the result of linking object files and libraries together to form an executable file. A typical program target looks like:

program: $(OBJS)
TAB echo Linking $@...
TAB $(CC) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)

Static Libraries

Static libraries have a prefix of “lib” and the extension “.a”. A typical static library target looks like:

libname.a: $(OBJECTS)
TAB echo Creating $@...
TAB $(RM) $@

Shared Libraries

Shared libraries have a prefix of “lib” and the extension “.dylib” or “.so” depending on the operating system. A typical shared library is composed of several targets that look like: $(OBJECTS)

libname.dylib: $(OBJECTS)
TAB echo $(DSOCOMMAND) libname.$(DSOVERSION).dylib ...
TAB $(DSOCOMMAND) libname.$(DSOVERSION).dylib \
TAB TAB -install_name $(libdir)/libname.$(DSOMAJOR).dylib \
TAB TAB -current_version libname.$(DSOVERSION).dylib \
TAB TAB -compatibility_version $(DSOMAJOR).0 \
TAB $(RM) libname.dylib
TAB $(RM) libname.$(DSOMAJOR).dylib
TAB $(LN) libname.$(DSOVERSION).dylib libname.$(DSOMAJOR).dylib
TAB $(LN) libname.$(DSOVERSION).dylib libname.dylib


Static dependencies are expressed in each makefile following the target, for example:

foo: bar

Static dependencies are only used when it is not possible to automatically generate them. Automatic dependencies are stored in a file named “Dependencies” and included at the end of the makefile. The following “depend” target rule is used to create the automatic dependencies:

TAB $(CC) -MM $(ALL_CFLAGS) $(OBJS:.o=.c) >Dependencies

We regenerate the automatic dependencies on an macOS system and express any non-macOS dependencies statically in the makefile.

Install/Uninstall Support

All makefiles contains install and uninstall rules which install or remove the corresponding software. These rules must use the $(BUILDROOT) variable as a prefix to any installation directory so that CUPS can be installed in a temporary location for packaging by programs like rpmbuild.

The INSTALL_BIN, INSTALL_COMPDATA, INSTALL_CONFIG, INSTALL_DATA, INSTALL_DIR, INSTALL_LIB, INSTALL_MAN, and INSTALL_SCRIPT variables must be used when installing files so that the proper ownership and permissions are set on the installed files.

The $(RANLIB) command must be run on any static libraries after installation since the symbol table is invalidated when the library is copied on some platforms.