Merge branch 'maint'
[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
15. The `buf` member is never NULL, so you it can be used in any usual C
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+
24However, it it totally safe to modify anything in the string pointed by
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
58This is string buffer structure. The `len` member can be used to
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
136NOTE: 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
163Add a NUL-terminated string to the buffer.
164+
165NOTE: This function will *always* be implemented as an inline or a macro
166that expands to:
167+
168----
169strbuf_add(..., s, strlen(s));
170----
171+
172Meaning that this is efficient to write things like:
173+
174----
175strbuf_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+
193The callback function is given a pointer to the character after the `%`
194and a pointer to the struct strbuf. It is expected to add the expanded
195version of the placeholder to the strbuf, e.g. to add a newline
196character if the letter `n` appears after a `%`. The function returns
197the length of the placeholder recognized and `strbuf_expand()` skips
198over it.
199+
200All other characters (non-percent and not skipped ones) are copied
201verbatim to the strbuf. If the callback returned zero, meaning that the
202placeholder is unknown, then the percent sign is copied, too.
203+
204In order to facilitate caching and to make it possible to give
205parameters to the callback, `strbuf_expand()` passes a context pointer,
206which can be used by the programmer of the callback as she sees fit.
207
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208`strbuf_expand_dict_cb`::
209
210 Used as callback for `strbuf_expand()`, expects an array of
211 struct strbuf_expand_dict_entry as context, i.e. pairs of
212 placeholder and replacement string. The array needs to be
213 terminated by an entry with placeholder set to NULL.
214
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215`strbuf_addf`::
216
217 Add a formatted string to the buffer.
218
219`strbuf_fread`::
220
221 Read a given size of data from a FILE* pointer to the buffer.
222+
223NOTE: The buffer is rewinded if the read fails. If -1 is returned,
224`errno` must be consulted, like you would do for `read(3)`.
225`strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the
226same behaviour as well.
227
228`strbuf_read`::
229
230 Read the contents of a given file descriptor. The third argument can be
231 used to give a hint about the file size, to avoid reallocs.
232
233`strbuf_read_file`::
234
235 Read the contents of a file, specified by its path. The third argument
236 can be used to give a hint about the file size, to avoid reallocs.
237
238`strbuf_getline`::
239
240 Read a line from a FILE* pointer. The second argument specifies the line
241 terminator character, typically `'\n'`.
242
243`stripspace`::
244
245 Strip whitespace from a buffer. The second parameter controls if
246 comments are considered contents to be removed or not.
247
248`launch_editor`::