Strbuf documentation: document most functions
[git/git.git] / Documentation / technical / api-strbuf.txt
1 strbuf API
2 ==========
3
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.
9
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.
12
13 strbufs has some invariants that are very important to keep in mind:
14
15 . The `buf` member is never NULL, so you 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 it totally safe to modify anything in the string pointed by
25 the `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 +
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.
52
53 Data structures
54 ---------------
55
56 * `struct strbuf`
57
58 This is 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.
61
62 Functions
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
136 NOTE: All of these functions in this section will grow the buffer as
137 necessary.
138
139 `strbuf_addch`::
140
141 Add a single character to the buffer.
142
143 `strbuf_insert`::
144
145 Insert data to the given position of the buffer. The remaining contents
146 will be shifted, not overwritten.
147
148 `strbuf_remove`::
149
150 Remove given amount of data from a given position of the buffer.
151
152 `strbuf_splice`::
153
154 Remove the bytes between `pos..pos+len` and replace it with the given
155 data.
156
157 `strbuf_add`::
158
159 Add data of given length to the buffer.
160
161 `strbuf_addstr`::
162
163 Add a NUL-terminated string to the buffer.
164 +
165 NOTE: This function will *always* be implemented as an inline or a macro
166 that expands to:
167 +
168 ----
169 strbuf_add(..., s, strlen(s));
170 ----
171 +
172 Meaning that this is efficient to write things like:
173 +
174 ----
175 strbuf_addstr(sb, "immediate string");
176 ----
177
178 `strbuf_addbuf`::
179
180 Copy the contents of an other buffer at the end of the current one.
181
182 `strbuf_adddup`::
183
184 Copy part of the buffer from a given position till a given length to the
185 end of the buffer.
186
187 `strbuf_expand`::
188
189 This function can be used to expand a format string containing
190 placeholders. To that end, it parses the string and calls the specified
191 function for every percent sign found.
192 +
193 The callback function is given a pointer to the character after the `%`
194 and a pointer to the struct strbuf. It is expected to add the expanded
195 version of the placeholder to the strbuf, e.g. to add a newline
196 character if the letter `n` appears after a `%`. The function returns
197 the length of the placeholder recognized and `strbuf_expand()` skips
198 over it.
199 +
200 All other characters (non-percent and not skipped ones) are copied
201 verbatim to the strbuf. If the callback returned zero, meaning that the
202 placeholder is unknown, then the percent sign is copied, too.
203 +
204 In order to facilitate caching and to make it possible to give
205 parameters to the callback, `strbuf_expand()` passes a context pointer,
206 which can be used by the programmer of the callback as she sees fit.
207
208 `strbuf_addf`::
209
210 Add a formatted string to the buffer.
211
212 `strbuf_fread`::
213
214 Read a given size of data from a FILE* pointer to the buffer.
215 +
216 NOTE: The buffer is rewinded if the read fails. If -1 is returned,
217 `errno` must be consulted, like you would do for `read(3)`.
218 `strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the
219 same behaviour as well.
220
221 `strbuf_read`::
222
223 Read the contents of a given file descriptor. The third argument can be
224 used to give a hint about the file size, to avoid reallocs.
225
226 `strbuf_read_file`::
227
228 Read the contents of a file, specified by its path. The third argument
229 can be used to give a hint about the file size, to avoid reallocs.
230
231 `strbuf_getline`::
232
233 Read a line from a FILE* pointer. The second argument specifies the line
234 terminator character, typically `'\n'`.
235
236 `stripspace`::
237
238 Strip whitespace from a buffer. The second parameter controls if
239 comments are considered contents to be removed or not.
240
241 `launch_editor`::