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[git/git.git] / Documentation / technical / api-strbuf.txt
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1strbuf API
2==========
3
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4strbuf's are meant to be used with all the usual C string and memory
5APIs. Given that the length of the buffer is known, it's often better to
6use the mem* functions than a str* one (memchr vs. strchr e.g.).
7Though, one has to be careful about the fact that str* functions often
8stop on NULs and that strbufs may have embedded NULs.
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10An strbuf is NUL terminated for convenience, but no function in the
11strbuf API actually relies on the string being free of NULs.
12
13strbufs has some invariants that are very important to keep in mind:
14
63b76948 15. The `buf` member is never NULL, so it can be used in any usual C
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16string operations safely. strbuf's _have_ to be initialized either by
17`strbuf_init()` or by `= STRBUF_INIT` before the invariants, though.
18+
19Do *not* assume anything on what `buf` really is (e.g. if it is
20allocated memory or not), use `strbuf_detach()` to unwrap a memory
21buffer from its strbuf shell in a safe way. That is the sole supported
22way. This will give you a malloced buffer that you can later `free()`.
23+
c9a8abcf 24However, it is totally safe to modify anything in the string pointed by
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25the `buf` member, between the indices `0` and `len-1` (inclusive).
26
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+
32NOTE: It is OK to "play" with the buffer directly if you work it this
33 way:
34+
35----
36strbuf_grow(sb, SOME_SIZE); <1>
37strbuf_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+
43NOTE: `SOME_OTHER_SIZE` must be smaller or equal to `strbuf_avail(sb)`.
44+
45Doing so is safe, though if it has to be done in many places, adding the
46missing API to the strbuf module is the way to go.
47+
48WARNING: 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()`
51instead.
52
53Data structures
54---------------
55
56* `struct strbuf`
57
63b76948 58This is the string buffer structure. The `len` member can be used to
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59determine the current length of the string, and `buf` member provides access to
60the string itself.
61
62Functions
63---------
64
65* Life cycle
66
67`strbuf_init`::
68
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.
71
72`strbuf_release`::
73
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.
76
77`strbuf_detach`::
78
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.
82
83`strbuf_attach`::
84
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.
91
92`strbuf_swap`::
93
94 Swap the contents of two string buffers.
95
96* Related to the size of the buffer
97
98`strbuf_avail`::
99
100 Determine the amount of allocated but unused memory.
101
102`strbuf_grow`::
103
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.
109
110`strbuf_setlen`::
111
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'.
117
118`strbuf_reset`::
119
120 Empty the buffer by setting the size of it to zero.
121
122* Related to the contents of the buffer
123
124`strbuf_rtrim`::
125
126 Strip whitespace from the end of a string.
127
128`strbuf_cmp`::
129
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.
133
134* Adding data to the buffer
135
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136NOTE: All of the functions in this section will grow the buffer as necessary.
137If they fail for some reason other than memory shortage and the buffer hadn't
138been allocated before (i.e. the `struct strbuf` was set to `STRBUF_INIT`),
139then they will free() it.
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140
141`strbuf_addch`::
142
143 Add a single character to the buffer.
144
145`strbuf_insert`::
146
147 Insert data to the given position of the buffer. The remaining contents
148 will be shifted, not overwritten.
149
150`strbuf_remove`::
151
152 Remove given amount of data from a given position of the buffer.
153
154`strbuf_splice`::
155
156 Remove the bytes between `pos..pos+len` and replace it with the given
157 data.
158
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159`strbuf_add_commented_lines`::
160
161 Add a NUL-terminated string to the buffer. Each line will be prepended
162 by a comment character and a blank.
163
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164`strbuf_add`::
165
166 Add data of given length to the buffer.
167
168`strbuf_addstr`::
169
170Add a NUL-terminated string to the buffer.
171+
172NOTE: This function will *always* be implemented as an inline or a macro
173that expands to:
174+
175----
176strbuf_add(..., s, strlen(s));
177----
178+
179Meaning that this is efficient to write things like:
180+
181----
182strbuf_addstr(sb, "immediate string");
183----
184
185`strbuf_addbuf`::
186
187 Copy the contents of an other buffer at the end of the current one.
188
189`strbuf_adddup`::
190
191 Copy part of the buffer from a given position till a given length to the
192 end of the buffer.
193
194`strbuf_expand`::
195
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+
200The callback function is given a pointer to the character after the `%`
201and a pointer to the struct strbuf. It is expected to add the expanded
202version of the placeholder to the strbuf, e.g. to add a newline
203character if the letter `n` appears after a `%`. The function returns
204the length of the placeholder recognized and `strbuf_expand()` skips
205over it.
206+
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207The format `%%` is automatically expanded to a single `%` as a quoting
208mechanism; callers do not need to handle the `%` placeholder themselves,
209and the callback function will not be invoked for this placeholder.
210+
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211All other characters (non-percent and not skipped ones) are copied
212verbatim to the strbuf. If the callback returned zero, meaning that the
213placeholder is unknown, then the percent sign is copied, too.
214+
215In order to facilitate caching and to make it possible to give
216parameters to the callback, `strbuf_expand()` passes a context pointer,
217which can be used by the programmer of the callback as she sees fit.
218
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219`strbuf_expand_dict_cb`::
220
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.
225
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226`strbuf_addbuf_percentquote`::
227
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.
232
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233`strbuf_addf`::
234
235 Add a formatted string to the buffer.
236
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237`strbuf_commented_addf`::
238
239 Add a formatted string prepended by a comment character and a
240 blank to the buffer.
241
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242`strbuf_fread`::
243
244 Read a given size of data from a FILE* pointer to the buffer.
245+
a1070d4c 246NOTE: The buffer is rewound if the read fails. If -1 is returned,
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247`errno` must be consulted, like you would do for `read(3)`.
248`strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the
249same behaviour as well.
250
251`strbuf_read`::
252
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.
255
256`strbuf_read_file`::
257
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.
260
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261`strbuf_readlink`::
262
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.
265
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266`strbuf_getline`::
267
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268 Read a line from a FILE *, overwriting the existing contents
269 of the strbuf. The second argument specifies the line
dd613e6b 270 terminator character, typically `'\n'`.
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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`.
274
275`strbuf_getwholeline`::
276
277 Like `strbuf_getline`, but keeps the trailing terminator (if
278 any) in the buffer.
279
280`strbuf_getwholeline_fd`::
281
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.
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286
287`stripspace`::
288
289 Strip whitespace from a buffer. The second parameter controls if
290 comments are considered contents to be removed or not.
291
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292`strbuf_split_buf`::
293`strbuf_split_str`::
294`strbuf_split_max`::
295`strbuf_split`::
296
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.
302
303`strbuf_list_free`::
304
305 Free a list of strbufs (for example, the return values of the
306 `strbuf_split()` functions).
307
dd613e6b 308`launch_editor`::
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309
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.