7438149249364ca8837811771b072b20990b3a5d
[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 is 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 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.
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
159 `strbuf_add`::
160
161 Add data of given length to the buffer.
162
163 `strbuf_addstr`::
164
165 Add a NUL-terminated string to the buffer.
166 +
167 NOTE: This function will *always* be implemented as an inline or a macro
168 that expands to:
169 +
170 ----
171 strbuf_add(..., s, strlen(s));
172 ----
173 +
174 Meaning that this is efficient to write things like:
175 +
176 ----
177 strbuf_addstr(sb, "immediate string");
178 ----
179
180 `strbuf_addbuf`::
181
182 Copy the contents of an other buffer at the end of the current one.
183
184 `strbuf_adddup`::
185
186 Copy part of the buffer from a given position till a given length to the
187 end of the buffer.
188
189 `strbuf_expand`::
190
191 This function can be used to expand a format string containing
192 placeholders. To that end, it parses the string and calls the specified
193 function for every percent sign found.
194 +
195 The callback function is given a pointer to the character after the `%`
196 and a pointer to the struct strbuf. It is expected to add the expanded
197 version of the placeholder to the strbuf, e.g. to add a newline
198 character if the letter `n` appears after a `%`. The function returns
199 the length of the placeholder recognized and `strbuf_expand()` skips
200 over it.
201 +
202 All other characters (non-percent and not skipped ones) are copied
203 verbatim to the strbuf. If the callback returned zero, meaning that the
204 placeholder is unknown, then the percent sign is copied, too.
205 +
206 In order to facilitate caching and to make it possible to give
207 parameters to the callback, `strbuf_expand()` passes a context pointer,
208 which can be used by the programmer of the callback as she sees fit.
209
210 `strbuf_expand_dict_cb`::
211
212 Used as callback for `strbuf_expand()`, expects an array of
213 struct strbuf_expand_dict_entry as context, i.e. pairs of
214 placeholder and replacement string. The array needs to be
215 terminated by an entry with placeholder set to NULL.
216
217 `strbuf_addf`::
218
219 Add a formatted string to the buffer.
220
221 `strbuf_fread`::
222
223 Read a given size of data from a FILE* pointer to the buffer.
224 +
225 NOTE: The buffer is rewound if the read fails. If -1 is returned,
226 `errno` must be consulted, like you would do for `read(3)`.
227 `strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the
228 same behaviour as well.
229
230 `strbuf_read`::
231
232 Read the contents of a given file descriptor. The third argument can be
233 used to give a hint about the file size, to avoid reallocs.
234
235 `strbuf_read_file`::
236
237 Read the contents of a file, specified by its path. The third argument
238 can be used to give a hint about the file size, to avoid reallocs.
239
240 `strbuf_readlink`::
241
242 Read the target of a symbolic link, specified by its path. The third
243 argument can be used to give a hint about the size, to avoid reallocs.
244
245 `strbuf_getline`::
246
247 Read a line from a FILE* pointer. The second argument specifies the line
248 terminator character, typically `'\n'`.
249
250 `stripspace`::
251
252 Strip whitespace from a buffer. The second parameter controls if
253 comments are considered contents to be removed or not.
254
255 `launch_editor`::