bundle: use a strbuf to scan the log for boundary commits
[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
159`strbuf_add`::
160
161 Add data of given length to the buffer.
162
163`strbuf_addstr`::
164
165Add a NUL-terminated string to the buffer.
166+
167NOTE: This function will *always* be implemented as an inline or a macro
168that expands to:
169+
170----
171strbuf_add(..., s, strlen(s));
172----
173+
174Meaning that this is efficient to write things like:
175+
176----
177strbuf_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+
195The callback function is given a pointer to the character after the `%`
196and a pointer to the struct strbuf. It is expected to add the expanded
197version of the placeholder to the strbuf, e.g. to add a newline
198character if the letter `n` appears after a `%`. The function returns
199the length of the placeholder recognized and `strbuf_expand()` skips
200over it.
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202The format `%%` is automatically expanded to a single `%` as a quoting
203mechanism; callers do not need to handle the `%` placeholder themselves,
204and the callback function will not be invoked for this placeholder.
205+
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206All other characters (non-percent and not skipped ones) are copied
207verbatim to the strbuf. If the callback returned zero, meaning that the
208placeholder is unknown, then the percent sign is copied, too.
209+
210In order to facilitate caching and to make it possible to give
211parameters to the callback, `strbuf_expand()` passes a context pointer,
212which can be used by the programmer of the callback as she sees fit.
213
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214`strbuf_expand_dict_cb`::
215
216 Used as callback for `strbuf_expand()`, expects an array of
217 struct strbuf_expand_dict_entry as context, i.e. pairs of
218 placeholder and replacement string. The array needs to be
219 terminated by an entry with placeholder set to NULL.
220
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221`strbuf_addbuf_percentquote`::
222
223 Append the contents of one strbuf to another, quoting any
224 percent signs ("%") into double-percents ("%%") in the
225 destination. This is useful for literal data to be fed to either
226 strbuf_expand or to the *printf family of functions.
227
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228`strbuf_addf`::
229
230 Add a formatted string to the buffer.
231
232`strbuf_fread`::
233
234 Read a given size of data from a FILE* pointer to the buffer.
235+
a1070d4c 236NOTE: The buffer is rewound if the read fails. If -1 is returned,
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237`errno` must be consulted, like you would do for `read(3)`.
238`strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the
239same behaviour as well.
240
241`strbuf_read`::
242
243 Read the contents of a given file descriptor. The third argument can be
244 used to give a hint about the file size, to avoid reallocs.
245
246`strbuf_read_file`::
247
248 Read the contents of a file, specified by its path. The third argument
249 can be used to give a hint about the file size, to avoid reallocs.
250
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251`strbuf_readlink`::
252
253 Read the target of a symbolic link, specified by its path. The third
254 argument can be used to give a hint about the size, to avoid reallocs.
255
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256`strbuf_getline`::
257
258 Read a line from a FILE* pointer. The second argument specifies the line
259 terminator character, typically `'\n'`.
260
261`stripspace`::
262
263 Strip whitespace from a buffer. The second parameter controls if
264 comments are considered contents to be removed or not.
265
266`launch_editor`::
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267
268 Launch the user preferred editor to edit a file and fill the buffer
269 with the file's contents upon the user completing their editing. The
270 third argument can be used to set the environment which the editor is
271 run in. If the buffer is NULL the editor is launched as usual but the
272 file's contents are not read into the buffer upon completion.