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Libgit2 Conventions

We like to keep the source consistent and readable. Herein are some guidelines that should help with that.

External API

We have a few rules to avoid surprising ways of calling functions and some rules for consumers of the library to avoid stepping on each other's toes.

  • Property accessors return the value directly (e.g. an int or const char *) but if a function can fail, we return a int value and the output parameters go first in the parameter list, followed by the object that a function is operating on, and then any other arguments the function may need.

  • If a function returns an object as a return value, that function is a getter and the object's lifetime is tied to the parent object. Objects which are returned as the first argument as a pointer-to-pointer are owned by the caller and it is responsible for freeing it. Strings are returned via git_buf in order to allow for re-use and safe freeing.

  • Most of what libgit2 does relates to I/O so as a general rule you should assume that any function can fail due to errors as even getting data from the filesystem can result in all sorts of errors and complex failure cases.

  • Paths inside the Git system are separated by a slash (0x2F). If a function accepts a path on disk, then backslashes (0x5C) are also accepted on Windows.

  • Do not mix allocators. If something has been allocated by libgit2, you do not know which is the right free function in the general case. Use the free functions provided for each object type.

Compatibility

libgit2 runs on many different platforms with many different compilers.

The public API of libgit2 is ANSI C (a.k.a. C89) compatible.

Internally, libgit2 is written using a portable subset of C99 - in order to maximize compatibility (e.g. with MSVC) we avoid certain C99 extensions. Specifically, we keep local variable declarations at the tops of blocks only and we avoid // style comments.

Also, to the greatest extent possible, we try to avoid lots of #ifdefs inside the core code base. This is somewhat unavoidable, but since it can really hamper maintainability, we keep it to a minimum.

Match Surrounding Code

If there is one rule to take away from this document, it is new code should match the surrounding code in a way that makes it impossible to distinguish the new from the old. Consistency is more important to us than anyone's personal opinion about where braces should be placed or spaces vs. tabs.

If a section of code is being completely rewritten, it is okay to bring it in line with the standards that are laid out here, but we will not accept submissions that contain a large number of changes that are merely reformatting.

Naming Things

All external types and functions start with git_ and all #define macros start with GIT_. The libgit2 API is mostly broken into related functional modules each with a corresponding header. All functions in a module should be named like git_modulename_functioname() (e.g. git_repository_open()).

Functions with a single output parameter should name that parameter out. Multiple outputs should be named foo_out, bar_out, etc.

Parameters of type git_oid should be named id, or foo_id. Calls that return an OID should be named git_foo_id.

Where a callback function is used, the function should also include a user-supplied extra input that is a void * named "payload" that will be passed through to the callback at each invocation.

Typedefs

Wherever possible, use typedef. In some cases, if a structure is just a collection of function pointers, the pointer types don't need to be separately typedef'd, but loose function pointer types should be.

Exports

All exported functions must be declared as:

GIT_EXTERN(result_type) git_modulename_functionname(arg_list);

Internals

Functions whose modulename is followed by two underscores, for example git_odb__read_packed, are semi-private functions. They are primarily intended for use within the library itself, and may disappear or change their signature in a future release.

Parameters

Out parameters come first.

Whenever possible, pass argument pointers as const. Some structures (such as git_repository and git_index) have mutable internal structure that prevents this.

Callbacks should always take a void * payload as their last parameter. Callback pointers are grouped with their payloads, and typically come last when passed as arguments:

int git_foo(git_repository *repo, git_foo_cb callback, void *payload);

Memory Ownership

Some APIs allocate memory which the caller is responsible for freeing; others return a pointer into a buffer that's owned by some other object. Make this explicit in the documentation.

Return codes

Most public APIs should return an int error code. As is typical with most C library functions, a zero value indicates success and a negative value indicates failure.

Some bindings will transform these returned error codes into exception types, so returning a semantically appropriate error code is important. Check include/git2/errors.h for the return codes already defined.

In your implementation, use git_error_set() to provide extended error information to callers.

If a libgit2 function internally invokes another function that reports an error, but the error is not propagated up, use git_error_clear() to prevent callers from getting the wrong error message later on.

Structs

Most public types should be opaque, e.g.:

typedef struct git_odb git_odb;

...with allocation functions returning an "instance" created within the library, and not within the application. This allows the type to grow (or shrink) in size without rebuilding client code.

To preserve ABI compatibility, include an int version field in all transparent structures, and initialize to the latest version in the constructor call. Increment the "latest" version whenever the structure changes, and try to only append to the end of the structure.

Option Structures

If a function's parameter count is too high, it may be desirable to package up the options in a structure. Make them transparent, include a version field, and provide an initializer constant or constructor. Using these structures should be this easy:

git_foo_options opts = GIT_FOO_OPTIONS_INIT;
opts.baz = BAZ_OPTION_ONE;
git_foo(&opts);

Enumerations

Typedef all enumerated types. If each option stands alone, use the enum type for passing them as parameters; if they are flags to be OR'ed together, pass them as unsigned int or uint32_t or some appropriate type.

Code Layout

Try to keep lines less than 80 characters long. This is a loose requirement, but going significantly over 80 columns is not nice.

Use common sense to wrap most code lines; public function declarations can use a couple of different styles:

/** All on one line is okay if it fits */
GIT_EXTERN(int) git_foo_simple(git_oid *id);

/** Otherwise one argument per line is a good next step */
GIT_EXTERN(int) git_foo_id(
	git_oid **out,
	int a,
	int b);

Indent with tabs; set your editor's tab width to eight for best effect.

Avoid trailing whitespace and only commit Unix-style newlines (i.e. no CRLF in the repository - just set core.autocrlf to true if you are writing code on a Windows machine).

Documentation

All comments should conform to Doxygen "javadoc" style conventions for formatting the public API documentation. Try to document every parameter, and keep the comments up to date if you change the parameter list.

Public Header Template

Use this template when creating a new public header.

#ifndef INCLUDE_git_${filename}_h__
#define INCLUDE_git_${filename}_h__

#include "git/common.h"

/**
 * @file git/${filename}.h
 * @brief Git some description
 * @defgroup git_${filename} some description routines
 * @ingroup Git
 * @{
 */
GIT_BEGIN_DECL

/* ... definitions ... */

/** @} */
GIT_END_DECL
#endif

Inlined functions

All inlined functions must be declared as:

GIT_INLINE(result_type) git_modulename_functionname(arg_list);

GIT_INLINE (or inline) should not be used in public headers in order to preserve ANSI C compatibility.

Tests

libgit2 uses the clar testing framework.

All PRs should have corresponding tests.

  • If the PR fixes an existing issue, the test should fail prior to applying the PR and succeed after applying it.
  • If the PR is for new functionality, then the tests should exercise that new functionality to a certain extent. We don't require 100% coverage right now (although we are getting stricter over time).

When adding new tests, we prefer if you attempt to reuse existing test data (in tests-clar/resources/) if possible. If you are going to add new test repositories, please try to strip them of unnecessary files (e.g. sample hooks, etc).