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_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
 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 +
 223 NOTE: 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
 226 same 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`::