Coding Style — The Mesa 3D Graphics Library latest documentation

# 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 Some old parts use a style that’s a bit out of date.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.