buffers.md

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Date : 2021-09-23T21:16:36

docs: document `git_buf`

We have been inconsistent about the way that we handle `git_buf`s
provided by users.  _Usually_ we require that it has been properly
initialized with `GIT_BUF_INIT`, but _sometimes_ we simply overwrite
the data in it regardless.  And even more rarely, we will grow a
user-provided buffer and concatenate data onto it (see
`git_diff_format_email`).

Document the path forward for `git_buf`, which is that we always
require that the buffer is intitialized with `GIT_BUF_INIT`.

`git_diff_format_email` will be kept backward compatible but users
are encouraged to switch to the new `git_email` APIs.

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Memory allocation and ownership

Any library needs to take data from users, and then return data to users. With some types this is simple - integer parameters and return types are trivial. But with more complex data types, things are more complicated. Even something seemingly simple, like a C string, requires discipline: we cannot simple return an allocated hunk of memory for callers to free, since some systems have multiple allocators and users cannot necessarily reason about the allocator used and which corresponding deallocation function to call to free the memory.

Objects

Most types in libgit2 are “opaque” types, which we treat as “objects” (even though C is “not an object oriented language”). You may create an object - for example, with git_odb_new, or libgit2 may return you an object as an “out” parameter - for example, with git_repository_open. With any of these objects, you should call their corresponding _free function (for example, git_odb_free or git_repository_free) when you are done using them.

Structures

libgit2 will often take input as structures (for example, options structures like git_merge_options). Rarely, libgit2 will return data in a structure. This is typically used for simpler data types, like git_buf and git_strarray. Users should allocate the structure themselves (either on the stack or the heap) and pass a pointer to it. Since libgit2 does not allocate the structure itself, only the data inside of it, the deallocation functions are suffixed with _dispose instead of _free, since they only free the data inside the structure.

Strings or continuous memory buffers (`git_buf`)

libgit2 typically takes NUL-terminated strings (“C strings”) with a const char *, and typically takes raw data with a const char * and a corresponding size_t for its size. libgit2 typically returns strings or raw data in a git_buf structure. The given data buffer will always be NUL terminated (even if it contains binary data) and the size member will always contain the size (in bytes) of the contents of the pointer (excluding the NUL terminator).

In other words, if a git_buf contains the string foo then the memory buffer will be { f, o, o, \0 } and the size will be 3.

Callers must initialize the buffer with GIT_BUF_INIT (or by setting all the members to 0) when it is created, before passing a pointer to the buffer to libgit2 for the first time.

Subsequent calls to libgit2 APIs that take a buffer can re-use a buffer that was previously used. The buffer will be cleared and grown to accommodate the new contents (if necessary). The new data will written into the buffer, overwriting the previous contents. This allows callers to reduce the number of allocations performed by the library.

Callers must call git_buf_dispose when they have finished.

Note that the deprecated git_diff_format_email API does not follow this behavior; subsequent calls will concatenate data to the buffer instead of rewriting it. Users should move to the new git_email APIs that follow the git_buf standards.

Source

docs/buffers.md

Memory allocation and ownership
-------------------------------

Any library needs to _take_ data from users, and then _return_ data to
users.  With some types this is simple - integer parameters and return
types are trivial.  But with more complex data types, things are more
complicated.  Even something seemingly simple, like a C string, requires
discipline: we cannot simple return an allocated hunk of memory for
callers to `free`, since some systems have multiple allocators and users
cannot necessarily reason about the allocator used and which corresponding
deallocation function to call to free the memory.

## Objects

Most types in libgit2 are "opaque" types, which we treat as "objects" (even
though C is "not an object oriented language").  You may create an object -
for example, with `git_odb_new`, or libgit2 may return you an object as an
"out" parameter - for example, with `git_repository_open`.  With any of
these objects, you should call their corresponding `_free` function (for
example, `git_odb_free` or `git_repository_free`) when you are done using
them.

## Structures

libgit2 will often take _input_ as structures (for example, options
structures like `git_merge_options`).  Rarely, libgit2 will return data in
a structure.  This is typically used for simpler data types, like `git_buf`
and `git_strarray`.  Users should allocate the structure themselves (either
on the stack or the heap) and pass a pointer to it.  Since libgit2 does not
allocate the structure itself, only the data inside of it, the deallocation
functions are suffixed with `_dispose` instead of `_free`, since they only
free the data _inside_ the structure.

## Strings or continuous memory buffers (`git_buf`)

libgit2 typically _takes_ NUL-terminated strings ("C strings") with a
`const char *`, and typically _takes_ raw data with a `const char *` and a
corresponding `size_t` for its size.  libgit2 typically _returns_ strings
or raw data in a `git_buf` structure.  The given data buffer will always be
NUL terminated (even if it contains binary data) and the `size` member will
always contain the size (in bytes) of the contents of the pointer (excluding
the NUL terminator).

In other words, if a `git_buf` contains the string `foo` then the memory
buffer will be { `f`, `o`, `o`, `\0` } and the size will be `3`.

Callers _must_ initialize the buffer with `GIT_BUF_INIT` (or by setting
all the members to `0`) when it is created, before passing a pointer
to the buffer to libgit2 for the first time.

Subsequent calls to libgit2 APIs that take a buffer can re-use a
buffer that was previously used.  The buffer will be cleared and grown
to accommodate the new contents (if necessary).  The new data will
written into the buffer, overwriting the previous contents.  This
allows callers to reduce the number of allocations performed by the
library.

Callers must call `git_buf_dispose` when they have finished.

Note that the deprecated `git_diff_format_email` API does not follow
this behavior; subsequent calls will concatenate data to the buffer
instead of rewriting it.  Users should move to the new `git_email`
APIs that follow the `git_buf` standards.