Coding Style¶

Mesa is over 20 years old and the coding style has evolved over time. Some old parts use a style that’s a bit out of date. Different sections of mesa can use different coding style as set in the local EditorConfig (.editorconfig) and/or Emacs (.dir-locals.el) file. Alternatively the following is applicable. If the guidelines below don’t cover something, try following the format of existing, neighboring code.

`clang-format`¶

A growing number of drivers and components are adopting clang-format to standardize the formatting and make it easy for everyone to apply it.

You can re-format the code for the components that have opted-in to the formatting enforcement (listed in .clang-format-include) by simply running ninja -C build/ clang-format.

Since mass-reformatting commits can be an annoying extra jump to go through when looking at git blame, you can configure it to ignore them by running:

git config blame.ignoreRevsFile .git-blame-ignore-revs

Most code editors also support automatically formatting code as you write it; check your editor or its plug-ins to see how to enable this.

Vim¶

Add this to your .vimrc to automatically format any C & C++ file (that has a .clang-format config) when you save it:

augroup ClangFormatOnSave
  au!

  function! ClangFormatOnSave()
    " Only format files that have a .clang-format in a parent folder
    if !empty(findfile('.clang-format', '.;'))
      let l:formatdiff = 1 " Only format lines that have changed
      py3f /usr/share/clang/clang-format.py
    endif
  endfunction

  autocmd BufWritePre *.h,*.c,*.cc,*.cpp call ClangFormatOnSave()
augroup END

If /usr/share/clang/clang-format.py doesn’t exist, try /usr/share/clang/clang-format-$CLANG_VERSION/clang-format.py (replacing $CLANG_VERSION with your clang version). If your distro has put the file somewhere else, look through the files in the package providing clang-format.

Emacs¶

Add this to your .emacs to automatically format any C & C++ file (that has a .clang-format config) when you save it:

(load "/usr/share/clang/clang-format.el")

(defun clang-format-save-hook-for-this-buffer ()
  "Create a buffer local save hook."
  (add-hook 'before-save-hook
            (lambda ()
              (when (locate-dominating-file "." ".clang-format")
                (clang-format-buffer))
              ;; Continue to save.
              nil)
            nil
            ;; Buffer local hook.
            t))

;; Run this for each mode you want to use the hook.
(add-hook 'c-mode-hook (lambda () (clang-format-save-hook-for-this-buffer)))
(add-hook 'c++-mode-hook (lambda () (clang-format-save-hook-for-this-buffer)))

If /usr/share/clang/clang-format.el doesn’t exist, look through the files in the package providing clang-format in your distro. If you can’t find anything (e.g. on Debian/Ubuntu), refer to this StackOverflow answer to install clang-format through Emacs instead.

git `pre-commit` hook¶

If your editor doesn’t support this, or if you don’t want to enable it, you can always just run ninja clang-format to format everything, or add a pre-commit hook that runs this automatically whenever you git commit by adding the following in your .git/hooks/pre-commit:

shopt -s globstar
git clang-format $upstream -- $(grep -E '^[^#]' .clang-format-include)
# replace $upstream with the name of the remote tracking upstream mesa
# if you don't know, it's probably `origin`

Basic formatting guidelines¶

3-space indentation, no tabs.
Limit lines to 78 or fewer characters. The idea is to prevent line wrapping in 80-column editors and terminals. There are exceptions, such as if you’re defining a large, static table of information.
Opening braces go on the same line as the if/for/while statement. For example:
```
if (condition) {
   foo;
} else {
   bar;
}
```
Put a space before/after operators. For example, a = b + c; and not a=b+c;
This GNU indent command generally does the right thing for formatting:
```
indent -br -i3 -npcs --no-tabs infile.c -o outfile.c
```

Use comments wherever you think it would be helpful for other developers. Several specific cases and style examples follow. Note that we roughly follow Doxygen conventions.

Single-line comments:

/* null-out pointer to prevent dangling reference below */
bufferObj = NULL;

Or,

bufferObj = NULL;  /* prevent dangling reference below */

Multi-line comment:

/* If this is a new buffer object id, or one which was generated but
 * never used before, allocate a buffer object now.
 */

We try to quote the OpenGL specification where prudent:

/* Page 38 of the PDF of the OpenGL ES 3.0 spec says:
 *
 *     "An INVALID_OPERATION error is generated for any of the following
 *     conditions:
 *
 *     * <length> is zero."
 *
 * Additionally, page 94 of the PDF of the OpenGL 4.5 core spec
 * (30.10.2014) also says this, so it's no longer allowed for desktop GL,
 * either.
 */

Function comment example:

/**
 * Create and initialize a new buffer object.  Called via the
 * ctx->Driver.CreateObject() driver callback function.
 * \param  name  integer name of the object
 * \param  type  one of GL_FOO, GL_BAR, etc.
 * \return  pointer to new object or NULL if error
 */
struct gl_object *
_mesa_create_object(GLuint name, GLenum type)
{
   /* function body */
}

Put the function return type and qualifiers on one line and the function name and parameters on the next, as seen above. This makes it easy to use grep ^function_name dir/* to find function definitions. Also, the opening brace goes on the next line by itself (see above.)

Function names follow various conventions depending on the type of function:

Convention	Explanation
`glFooBar()`	a public GL entry point (in `glapi_dispatch.c`)
`_mesa_FooBar()`	the internal immediate mode function
`save_FooBar()`	retained mode (display list) function in `dlist.c`
`foo_bar()`	a static (private) function
`_mesa_foo_bar()`	an internal non-static Mesa function

Constants, macros and enum names are ALL_UPPERCASE, with _ between words.
Mesa usually uses camel case for local variables (Ex: localVarname) while Gallium typically uses underscores (Ex: local_var_name).
Global variables are almost never used because Mesa should be thread-safe.
Booleans. Places that are not directly visible to the GL API should prefer the use of bool, true, and false over GLboolean, GL_TRUE, and GL_FALSE. In C code, this may mean that #include <stdbool.h> needs to be added. The try_emit_* method src/mesa/state_tracker/st_glsl_to_tgsi.cpp can serve as an example.