path: root/Documentation/technical/api-strbuf.txt

strbuf API
==========

strbuf's are meant to be used with all the usual C string and memory
APIs. Given that the length of the buffer is known, it's often better to
use the mem* functions than a str* one (memchr vs. strchr e.g.).
Though, one has to be careful about the fact that str* functions often
stop on NULs and that strbufs may have embedded NULs.

An strbuf is NUL terminated for convenience, but no function in the
strbuf API actually relies on the string being free of NULs.

strbufs has some invariants that are very important to keep in mind:

. The `buf` member is never NULL, so it can be used in any usual C
string operations safely. strbuf's _have_ to be initialized either by
`strbuf_init()` or by `= STRBUF_INIT` before the invariants, though.
+
Do *not* assume anything on what `buf` really is (e.g. if it is
allocated memory or not), use `strbuf_detach()` to unwrap a memory
buffer from its strbuf shell in a safe way. That is the sole supported
way. This will give you a malloced buffer that you can later `free()`.
+
However, it is totally safe to modify anything in the string pointed by
the `buf` member, between the indices `0` and `len-1` (inclusive).

. The `buf` member is a byte array that has at least `len + 1` bytes
  allocated. The extra byte is used to store a `'\0'`, allowing the
  `buf` member to be a valid C-string. Every strbuf function ensure this
  invariant is preserved.
+
NOTE: It is OK to "play" with the buffer directly if you work it this
      way:
+
----
strbuf_grow(sb, SOME_SIZE); <1>
strbuf_setlen(sb, sb->len + SOME_OTHER_SIZE);
----
<1> Here, the memory array starting at `sb->buf`, and of length
`strbuf_avail(sb)` is all yours, and you can be sure that
`strbuf_avail(sb)` is at least `SOME_SIZE`.
+
NOTE: `SOME_OTHER_SIZE` must be smaller or equal to `strbuf_avail(sb)`.
+
Doing so is safe, though if it has to be done in many places, adding the
missing API to the strbuf module is the way to go.
+
WARNING: Do _not_ assume that the area that is yours is of size `alloc
- 1` even if it's true in the current implementation. Alloc is somehow a
"private" member that should not be messed with. Use `strbuf_avail()`
instead.

Data structures
---------------

* `struct strbuf`

This is the string buffer structure. The `len` member can be used to
determine the current length of the string, and `buf` member provides access to
the string itself.

Functions
---------

* Life cycle

`strbuf_init`::

	Initialize the structure. The second parameter can be zero or a bigger
	number to allocate memory, in case you want to prevent further reallocs.

`strbuf_release`::

	Release a string buffer and the memory it used. You should not use the
	string buffer after using this function, unless you initialize it again.

`strbuf_detach`::

	Detach the string from the strbuf and returns it; you now own the
	storage the string occupies and it is your responsibility from then on
	to release it with `free(3)` when you are done with it.

`strbuf_attach`::

	Attach a string to a buffer. You should specify the string to attach,
	the current length of the string and the amount of allocated memory.
	The amount must be larger than the string length, because the string you
	pass is supposed to be a NUL-terminated string.  This string _must_ be
	malloc()ed, and after attaching, the pointer cannot be relied upon
	anymore, and neither be free()d directly.

`strbuf_swap`::

	Swap the contents of two string buffers.

* Related to the size of the buffer

`strbuf_avail`::

	Determine the amount of allocated but unused memory.

`strbuf_grow`::

	Ensure that at least this amount of unused memory is available after
	`len`. This is used when you know a typical size for what you will add
	and want to avoid repetitive automatic resizing of the underlying buffer.
	This is never a needed operation, but can be critical for performance in
	some cases.

`strbuf_setlen`::

	Set the length of the buffer to a given value. This function does *not*
	allocate new memory, so you should not perform a `strbuf_setlen()` to a
	length that is larger than `len + strbuf_avail()`. `strbuf_setlen()` is
	just meant as a 'please fix invariants from this strbuf I just messed
	with'.

`strbuf_reset`::

	Empty the buffer by setting the size of it to zero.

* Related to the contents of the buffer

`strbuf_rtrim`::

	Strip whitespace from the end of a string.

`strbuf_cmp`::

	Compare two buffers. Returns an integer less than, equal to, or greater
	than zero if the first buffer is found, respectively, to be less than,
	to match, or be greater than the second buffer.

* Adding data to the buffer

NOTE: All of the functions in this section will grow the buffer as necessary.
If they fail for some reason other than memory shortage and the buffer hadn't
been allocated before (i.e. the `struct strbuf` was set to `STRBUF_INIT`),
then they will free() it.

`strbuf_addch`::

	Add a single character to the buffer.

`strbuf_insert`::

	Insert data to the given position of the buffer. The remaining contents
	will be shifted, not overwritten.

`strbuf_remove`::

	Remove given amount of data from a given position of the buffer.

`strbuf_splice`::

	Remove the bytes between `pos..pos+len` and replace it with the given
	data.

`strbuf_add`::

	Add data of given length to the buffer.

`strbuf_addstr`::

Add a NUL-terminated string to the buffer.
+
NOTE: This function will *always* be implemented as an inline or a macro
that expands to:
+
----
strbuf_add(..., s, strlen(s));
----
+
Meaning that this is efficient to write things like:
+
----
strbuf_addstr(sb, "immediate string");
----

`strbuf_addbuf`::

	Copy the contents of an other buffer at the end of the current one.

`strbuf_adddup`::

	Copy part of the buffer from a given position till a given length to the
	end of the buffer.

`strbuf_expand`::

	This function can be used to expand a format string containing
	placeholders. To that end, it parses the string and calls the specified
	function for every percent sign found.
+
The callback function is given a pointer to the character after the `%`
and a pointer to the struct strbuf.  It is expected to add the expanded
version of the placeholder to the strbuf, e.g. to add a newline
character if the letter `n` appears after a `%`.  The function returns
the length of the placeholder recognized and `strbuf_expand()` skips
over it.
+
The format `%%` is automatically expanded to a single `%` as a quoting
mechanism; callers do not need to handle the `%` placeholder themselves,
and the callback function will not be invoked for this placeholder.
+
All other characters (non-percent and not skipped ones) are copied
verbatim to the strbuf.  If the callback returned zero, meaning that the
placeholder is unknown, then the percent sign is copied, too.
+
In order to facilitate caching and to make it possible to give
parameters to the callback, `strbuf_expand()` passes a context pointer,
which can be used by the programmer of the callback as she sees fit.

`strbuf_expand_dict_cb`::

	Used as callback for `strbuf_expand()`, expects an array of
	struct strbuf_expand_dict_entry as context, i.e. pairs of
	placeholder and replacement string.  The array needs to be
	terminated by an entry with placeholder set to NULL.

`strbuf_addbuf_percentquote`::

	Append the contents of one strbuf to another, quoting any
	percent signs ("%") into double-percents ("%%") in the
	destination. This is useful for literal data to be fed to either
	strbuf_expand or to the *printf family of functions.

`strbuf_addf`::

	Add a formatted string to the buffer.

`strbuf_fread`::

	Read a given size of data from a FILE* pointer to the buffer.
+
NOTE: The buffer is rewound if the read fails. If -1 is returned,
`errno` must be consulted, like you would do for `read(3)`.
`strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the
same behaviour as well.

`strbuf_read`::

	Read the contents of a given file descriptor. The third argument can be
	used to give a hint about the file size, to avoid reallocs.

`strbuf_read_file`::

	Read the contents of a file, specified by its path. The third argument
	can be used to give a hint about the file size, to avoid reallocs.

`strbuf_readlink`::

	Read the target of a symbolic link, specified by its path.  The third
	argument can be used to give a hint about the size, to avoid reallocs.

`strbuf_getline`::

	Read a line from a FILE* pointer. The second argument specifies the line
	terminator character, typically `'\n'`.

`stripspace`::

	Strip whitespace from a buffer. The second parameter controls if
	comments are considered contents to be removed or not.

`launch_editor`::

	Launch the user preferred editor to edit a file and fill the buffer
	with the file's contents upon the user completing their editing. The
	third argument can be used to set the environment which the editor is
	run in. If the buffer is NULL the editor is launched as usual but the
	file's contents are not read into the buffer upon completion.