Submitting C code

When submitting C code to GRASS GIS GitHub repository, please take care of following rules (see also RFC/7_LanguageStandardsSupport):

API Manual

Get and read the GRASS Programmer's Manual here:

Or generate it from this source code (the programmer's manual is integrated in the source code in doxygen style):

      make htmldocs
      make pdfdocs

Directory Conventions

Use the directory structure to place your module appropriately into the source tree

  • libraries go into lib/
  • raster modules go into raster/
  • vector modules go into vector/
  • ...

Consider to take a look at "GNU Coding Standards":


Add a header section to file main.c of your module and make sure you include the copyright. The purpose section is meant to contain a general overview of the code in the file to assist other programmers that will need to make changes to your code. If you are modifying an existing file you may under no circumstances remove prior copyright or licensing text that is not your own, even for a major rewrite. If any original code or code that is in part derived from another's original work remains, it must be properly cited.


 * AUTHOR(S):    John Doe <jdoe at somewhere org>
 * PURPOSE:      Provide short description of module here...
 * COPYRIGHT:    (C) 2010 by John Doe, and the GRASS Development Team
 *               This program is free software under the GNU General Public
 *               License (>=v2). Read the COPYING file that comes with GRASS
 *               for details.

The copyright protects your rights according to GNU General Public License (

GRASS Config Header

To ensure that the software system continues to work, please include

	#include <grass/config.h>

in your files and make use of the various system dependencies contained therein. As one example of this, see source:grass/trunk/lib/gmath/fft.c. Please refrain from declaring system functions within the software; include the proper header files (conditionally dependent on config.h macros if necessary) instead.

Other Headers

Order of include headers

In general, headers should be included in the order:

  1. Core system headers (stdio.h, ctype.h, ...)
  2. Headers for non-core system components (X11, libraries).
  3. Headers for core systems of the package being compiled (grass/gis.h, grass/glocale.h, ...)
  4. Headers for the specific library/program being compiled (geodesic.h, ...)

Each class of header has an obligation to be compatible with those above it in the list, but not those below it.



Always specify the return type for ALL functions including those that return type "void", and insert return statements for any function which returns a value.

Also, use ANSI C prototypes to declare your functions. For module return values, see "Exit status" below.


    void G_something(void);
    int G_something_else(int, int);
    void G_something(void)
    	/* Snipped out code */
    int G_something_else(int x, int y)
    	/* Snipped out code */
	return 0;


Use the GRASS library function G_asprintf() instead of the standard C functions asprintf(), vsnprintf() and snprintf(). These functions are not portable or have other issues. Example:

    char *msg;

    G_asprintf(&msg, "%s", parameters);

Note that you should free memory when G_asprintf() is used.

Memory Allocations

Use the following GRASS library functions instead of the standard C functions. The reason for this is that the following functions ensure good programming practice (e.g. always checking if memory was allocated) and/or improves portability. PLEASE refer to the programmers manual for the proper use (e.g. determining if any casts are needed for arguments or return values) of these library functions. They may perform a task slightly different from their corresponding C library function, and thus, their use may not be the same.

    	G_malloc() instead of malloc()
	G_calloc() instead of calloc()
	G_realloc() instead of realloc()
	G_free() instead of free()
	G_getenv() instead of getenv()
	G_setenv() instead of setenv()
	G_unsetenv() instead of unsetenv()
	G_sleep() instead of sleep()

Could somebody please add others (please verify that they are useful and safe first)

Naming Conventions

Use function names which fulfill the official GNU naming convention:

Instead of naming a function like: MyNewFunction() use underscores for seperation and lower case letters: my_new_function()`.


If you want to comment code portions, use

       #ifdef notdef 

This is safe comparing to nested /* comments */

Functions in the library must be documented in doxygen style to get them into the programmer's manual (generate with make pdfdocs or make htmldocs). See source:grass/trunk/lib/gis/ for examples.

Documentation in Doxygen

Use doxygen style for source code documentation. It is required for GRASS libraries, but also recommended for GRASS modules.

Do not use structural command inside documentation block since it leads to some duplication of information (e.g. do not use \fn command in comment blocks). The exception is \file command for documenting a file, in this case structural command is required.

For files

       \file snap.c
       \brief Vector library - Clean vector map (snap lines)

       (C) 2001-2008 by the GRASS Development Team
       This program is free software under the GNU General Public
       License (>=v2).  Read the file COPYING that comes with GRASS
       for details.
       \author Radim Blazek

For functions

       \brief Snap lines in vector map to existing vertex in threshold

       For details see Vect_snap_lines_list()

       \param Map pointer to input vector map 
       \param type filter features of given type to be snap
       \param thresh threshold value for snapping
       \param[out] Err pointer to vector map where lines representing snap are written or NULL
       \param[out] msgout file pointer where messages will be written or NULL
       \return 1	   


Returning Value Of Main function

Module exit status is defined as EXIT_SUCCESS or EXIT_FAILURE (declared in stdlib.h), e.g.

      if (G_parser (argc, argv))
          exit (EXIT_FAILURE);

      exit (EXIT_SUCCESS);


Use fprintf() instead of printf(). For errors and warnings please use the G_fatal_error() and G_warning() functions. General messages for the user should use G_message() while debug messages should use G_debug() whenever possible.

There are two variants to G_message(): G_verbose_message() which will only display the message if in --verbose mode, and G_important_message() which will always show the message unless the module is running in --quiet mode. G_fatal_error() and G_warning() will always be displayed regardless of verbosity setting. Messages sent to any of these functions will be printed to stderr.

G_message() output is not expected to be sent to pipe or file.

Always use the gettext macros with _("") for user messages, example:

      G_fatal_error(_("Vector map <%s> not found"), name); 

It is suggested to add a comment line before translatable user message to give a hint to translators about meaning or use of cumbersome or obscure message. First word in the comment must be GTC - GRASS translation comment,


        /* GTC A name of a projection */
        G_message(_("State Plane"));

Any message with a noun in plural form has to pass n_() macro, even if for the English language it is not required!

       G_message(n_("One map", "%d maps", number), number);

See source:grass/trunk/locale/README for details.

Pipe/file data output: For data output redirected to pipe or file, please use fprintf() and specify the stdout stream as follows:

      fprintf(stdout, ...);

      fflush(stdout) /* always required when using fprintf(stdout, ...). */

Map History

Have a function included in your module which writes to the history file of the map (e.g. command line, parameters etc.). See e.g. source:grass/trunk/raster/r.patch/main.c (the same applies to vector and raster3d modules!)

Standardized Options and Flags

Standard parser options: use G_define_standard_option() whenever possible to define standard module command line options. This will save you time, create fewer bugs, and make things easier on the translators. See source:grass/trunk/lib/gis/parser_standard_options.c for details of the function definition.

Adding description and keywords

Each module needs to have a description and at least 3 keywords. Here an example from general/g.list/main.c:


    module = G_define_module();
    G_add_keyword(_("map management"));
    module->description =
        _("Lists available GRASS data base files of "
          "the user-specified data type optionally using the search pattern.");


  • the first keyword is the module family (g.list belongs to "general") which go to the module family index in the manual
  • the second keyword is the overall topic which go to the topic index in the manual
  • the third* (and more) keyword is describing further keywords which go to the keyword index in the manual

These index manual pages are autogenerated during the build process of GRASS GIS.


To promote a consistent coding style for whitespace (spaces, tabs, newlines), please use the indent tool on all new C files using the following switches:

-bad -bap -bbb -br -bli0 -bls -cli0 -ncs -fc1 -hnl -i4
-nbbo -nbc -nbfda -nbfde -ncdb -ncdw -nce -nfca -npcs -nprs
-npsl -nsc -nsob -saf -sai -saw -sbi0 -ss --no-tabs

Existing code should not be re-indented as this will make "diff" comparisons with older versions impossible. In extreme cases, when run of indent tool is necessary, do not commit (check in) any changes other than the indentation in the same commit. Additionally, add the indent tool switches and any indent warning messages to the commit message (git log). Any change or fix mixed in with an indentation is very hard to track making it hard for others to follow the change, fix any new bugs or track the origin of particular lines (git diff --ignore-space-change is helpful in this case, but does not solve the problem).

For your convenience use the script available in GRASS source code.

See GRASS Wiki or man indent for explanation of individual switches.


Platform dependent code:

Do not remove #ifdef __CYGWIN__ and/or #ifndef __CYGWIN__ lines and their encapsulated lines from source code (one example was that someone removed drand48 definition.)

Compiler Flags

Suggested compiler flags: We suggest to use very strict compiler flags to capture errors at the very beginning. Here our list of flags, please use them to configure you development version of GRASS GIS.

See also


       MYCFLAGS="-g -Wall -Werror-implicit-function-declaration -Wreturn-type -fno-common -fexceptions"
       MYCXXFLAGS="-g -Wall"
       MYLDFLAGS="-Wl,--no-undefined -Wl,-z,relro"



(to be suggested, see macosx/ReadMe.rtf)


(to be suggested, see CompileOnWindows)

FreeBSD / NetBSD

(to be suggested)

See also


(to be suggested)

See also


(to be suggested)

See also


Last modified 8 months ago Last modified on Feb 6, 2022, 3:11:42 AM
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