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[git/git.git] / Documentation / technical / api-strbuf.txt
1 strbuf API
2 ==========
4 strbuf's are meant to be used with all the usual C string and memory
5 APIs. Given that the length of the buffer is known, it's often better to
6 use the mem* functions than a str* one (memchr vs. strchr e.g.).
7 Though, one has to be careful about the fact that str* functions often
8 stop on NULs and that strbufs may have embedded NULs.
10 An strbuf is NUL terminated for convenience, but no function in the
11 strbuf API actually relies on the string being free of NULs.
13 strbufs has some invariants that are very important to keep in mind:
15 . The `buf` member is never NULL, so it can be used in any usual C
16 string operations safely. strbuf's _have_ to be initialized either by
17 `strbuf_init()` or by `= STRBUF_INIT` before the invariants, though.
18 +
19 Do *not* assume anything on what `buf` really is (e.g. if it is
20 allocated memory or not), use `strbuf_detach()` to unwrap a memory
21 buffer from its strbuf shell in a safe way. That is the sole supported
22 way. This will give you a malloced buffer that you can later `free()`.
23 +
24 However, it is totally safe to modify anything in the string pointed by
25 the `buf` member, between the indices `0` and `len-1` (inclusive).
27 . The `buf` member is a byte array that has at least `len + 1` bytes
28 allocated. The extra byte is used to store a `'\0'`, allowing the
29 `buf` member to be a valid C-string. Every strbuf function ensure this
30 invariant is preserved.
31 +
32 NOTE: It is OK to "play" with the buffer directly if you work it this
33 way:
34 +
35 ----
36 strbuf_grow(sb, SOME_SIZE); <1>
37 strbuf_setlen(sb, sb->len + SOME_OTHER_SIZE);
38 ----
39 <1> Here, the memory array starting at `sb->buf`, and of length
40 `strbuf_avail(sb)` is all yours, and you can be sure that
41 `strbuf_avail(sb)` is at least `SOME_SIZE`.
42 +
43 NOTE: `SOME_OTHER_SIZE` must be smaller or equal to `strbuf_avail(sb)`.
44 +
45 Doing so is safe, though if it has to be done in many places, adding the
46 missing API to the strbuf module is the way to go.
47 +
48 WARNING: Do _not_ assume that the area that is yours is of size `alloc
49 - 1` even if it's true in the current implementation. Alloc is somehow a
50 "private" member that should not be messed with. Use `strbuf_avail()`
51 instead.
53 Data structures
54 ---------------
56 * `struct strbuf`
58 This is the string buffer structure. The `len` member can be used to
59 determine the current length of the string, and `buf` member provides access to
60 the string itself.
62 Functions
63 ---------
65 * Life cycle
67 `strbuf_init`::
69 Initialize the structure. The second parameter can be zero or a bigger
70 number to allocate memory, in case you want to prevent further reallocs.
72 `strbuf_release`::
74 Release a string buffer and the memory it used. You should not use the
75 string buffer after using this function, unless you initialize it again.
77 `strbuf_detach`::
79 Detach the string from the strbuf and returns it; you now own the
80 storage the string occupies and it is your responsibility from then on
81 to release it with `free(3)` when you are done with it.
83 `strbuf_attach`::
85 Attach a string to a buffer. You should specify the string to attach,
86 the current length of the string and the amount of allocated memory.
87 The amount must be larger than the string length, because the string you
88 pass is supposed to be a NUL-terminated string. This string _must_ be
89 malloc()ed, and after attaching, the pointer cannot be relied upon
90 anymore, and neither be free()d directly.
92 `strbuf_swap`::
94 Swap the contents of two string buffers.
96 * Related to the size of the buffer
98 `strbuf_avail`::
100 Determine the amount of allocated but unused memory.
102 `strbuf_grow`::
104 Ensure that at least this amount of unused memory is available after
105 `len`. This is used when you know a typical size for what you will add
106 and want to avoid repetitive automatic resizing of the underlying buffer.
107 This is never a needed operation, but can be critical for performance in
108 some cases.
110 `strbuf_setlen`::
112 Set the length of the buffer to a given value. This function does *not*
113 allocate new memory, so you should not perform a `strbuf_setlen()` to a
114 length that is larger than `len + strbuf_avail()`. `strbuf_setlen()` is
115 just meant as a 'please fix invariants from this strbuf I just messed
116 with'.
118 `strbuf_reset`::
120 Empty the buffer by setting the size of it to zero.
122 * Related to the contents of the buffer
124 `strbuf_rtrim`::
126 Strip whitespace from the end of a string.
128 `strbuf_cmp`::
130 Compare two buffers. Returns an integer less than, equal to, or greater
131 than zero if the first buffer is found, respectively, to be less than,
132 to match, or be greater than the second buffer.
134 * Adding data to the buffer
136 NOTE: All of the functions in this section will grow the buffer as necessary.
137 If they fail for some reason other than memory shortage and the buffer hadn't
138 been allocated before (i.e. the `struct strbuf` was set to `STRBUF_INIT`),
139 then they will free() it.
141 `strbuf_addch`::
143 Add a single character to the buffer.
145 `strbuf_insert`::
147 Insert data to the given position of the buffer. The remaining contents
148 will be shifted, not overwritten.
150 `strbuf_remove`::
152 Remove given amount of data from a given position of the buffer.
154 `strbuf_splice`::
156 Remove the bytes between `pos..pos+len` and replace it with the given
157 data.
159 `strbuf_add_commented_lines`::
161 Add a NUL-terminated string to the buffer. Each line will be prepended
162 by a comment character and a blank.
164 `strbuf_add`::
166 Add data of given length to the buffer.
168 `strbuf_addstr`::
170 Add a NUL-terminated string to the buffer.
171 +
172 NOTE: This function will *always* be implemented as an inline or a macro
173 that expands to:
174 +
175 ----
176 strbuf_add(..., s, strlen(s));
177 ----
178 +
179 Meaning that this is efficient to write things like:
180 +
181 ----
182 strbuf_addstr(sb, "immediate string");
183 ----
185 `strbuf_addbuf`::
187 Copy the contents of an other buffer at the end of the current one.
189 `strbuf_adddup`::
191 Copy part of the buffer from a given position till a given length to the
192 end of the buffer.
194 `strbuf_expand`::
196 This function can be used to expand a format string containing
197 placeholders. To that end, it parses the string and calls the specified
198 function for every percent sign found.
199 +
200 The callback function is given a pointer to the character after the `%`
201 and a pointer to the struct strbuf. It is expected to add the expanded
202 version of the placeholder to the strbuf, e.g. to add a newline
203 character if the letter `n` appears after a `%`. The function returns
204 the length of the placeholder recognized and `strbuf_expand()` skips
205 over it.
206 +
207 The format `%%` is automatically expanded to a single `%` as a quoting
208 mechanism; callers do not need to handle the `%` placeholder themselves,
209 and the callback function will not be invoked for this placeholder.
210 +
211 All other characters (non-percent and not skipped ones) are copied
212 verbatim to the strbuf. If the callback returned zero, meaning that the
213 placeholder is unknown, then the percent sign is copied, too.
214 +
215 In order to facilitate caching and to make it possible to give
216 parameters to the callback, `strbuf_expand()` passes a context pointer,
217 which can be used by the programmer of the callback as she sees fit.
219 `strbuf_expand_dict_cb`::
221 Used as callback for `strbuf_expand()`, expects an array of
222 struct strbuf_expand_dict_entry as context, i.e. pairs of
223 placeholder and replacement string. The array needs to be
224 terminated by an entry with placeholder set to NULL.
226 `strbuf_addbuf_percentquote`::
228 Append the contents of one strbuf to another, quoting any
229 percent signs ("%") into double-percents ("%%") in the
230 destination. This is useful for literal data to be fed to either
231 strbuf_expand or to the *printf family of functions.
233 `strbuf_addf`::
235 Add a formatted string to the buffer.
237 `strbuf_commented_addf`::
239 Add a formatted string prepended by a comment character and a
240 blank to the buffer.
242 `strbuf_fread`::
244 Read a given size of data from a FILE* pointer to the buffer.
245 +
246 NOTE: The buffer is rewound if the read fails. If -1 is returned,
247 `errno` must be consulted, like you would do for `read(3)`.
248 `strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the
249 same behaviour as well.
251 `strbuf_read`::
253 Read the contents of a given file descriptor. The third argument can be
254 used to give a hint about the file size, to avoid reallocs.
256 `strbuf_read_file`::
258 Read the contents of a file, specified by its path. The third argument
259 can be used to give a hint about the file size, to avoid reallocs.
261 `strbuf_readlink`::
263 Read the target of a symbolic link, specified by its path. The third
264 argument can be used to give a hint about the size, to avoid reallocs.
266 `strbuf_getline`::
268 Read a line from a FILE *, overwriting the existing contents
269 of the strbuf. The second argument specifies the line
270 terminator character, typically `'\n'`.
271 Reading stops after the terminator or at EOF. The terminator
272 is removed from the buffer before returning. Returns 0 unless
273 there was nothing left before EOF, in which case it returns `EOF`.
275 `strbuf_getwholeline`::
277 Like `strbuf_getline`, but keeps the trailing terminator (if
278 any) in the buffer.
280 `strbuf_getwholeline_fd`::
282 Like `strbuf_getwholeline`, but operates on a file descriptor.
283 It reads one character at a time, so it is very slow. Do not
284 use it unless you need the correct position in the file
285 descriptor.
287 `stripspace`::
289 Strip whitespace from a buffer. The second parameter controls if
290 comments are considered contents to be removed or not.
292 `strbuf_split_buf`::
293 `strbuf_split_str`::
294 `strbuf_split_max`::
295 `strbuf_split`::
297 Split a string or strbuf into a list of strbufs at a specified
298 terminator character. The returned substrings include the
299 terminator characters. Some of these functions take a `max`
300 parameter, which, if positive, limits the output to that
301 number of substrings.
303 `strbuf_list_free`::
305 Free a list of strbufs (for example, the return values of the
306 `strbuf_split()` functions).
308 `launch_editor`::
310 Launch the user preferred editor to edit a file and fill the buffer
311 with the file's contents upon the user completing their editing. The
312 third argument can be used to set the environment which the editor is
313 run in. If the buffer is NULL the editor is launched as usual but the
314 file's contents are not read into the buffer upon completion.