Documentation / technical / api-strbuf.txton commit Merge branch 'mb/local-clone-after-applying-insteadof' (bc88def)
   1strbuf API
   2==========
   3
   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.
   9
  10A strbuf is NUL terminated for convenience, but no function in the
  11strbuf API actually relies on the string being free of NULs.
  12
  13strbufs have some invariants that are very important to keep in mind:
  14
  15. The `buf` member is never NULL, so 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 is 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 the string buffer structure. The `len` member can be used to
  59determine the current length of the string, and `buf` member provides
  60access to the 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_trim`::
 125
 126        Strip whitespace from the beginning and end of a string.
 127        Equivalent to performing `strbuf_rtrim()` followed by `strbuf_ltrim()`.
 128
 129`strbuf_rtrim`::
 130
 131        Strip whitespace from the end of a string.
 132
 133`strbuf_ltrim`::
 134
 135        Strip whitespace from the beginning of a string.
 136
 137`strbuf_reencode`::
 138
 139        Replace the contents of the strbuf with a reencoded form.  Returns -1
 140        on error, 0 on success.
 141
 142`strbuf_tolower`::
 143
 144        Lowercase each character in the buffer using `tolower`.
 145
 146`strbuf_cmp`::
 147
 148        Compare two buffers. Returns an integer less than, equal to, or greater
 149        than zero if the first buffer is found, respectively, to be less than,
 150        to match, or be greater than the second buffer.
 151
 152* Adding data to the buffer
 153
 154NOTE: All of the functions in this section will grow the buffer as necessary.
 155If they fail for some reason other than memory shortage and the buffer hadn't
 156been allocated before (i.e. the `struct strbuf` was set to `STRBUF_INIT`),
 157then they will free() it.
 158
 159`strbuf_addch`::
 160
 161        Add a single character to the buffer.
 162
 163`strbuf_insert`::
 164
 165        Insert data to the given position of the buffer. The remaining contents
 166        will be shifted, not overwritten.
 167
 168`strbuf_remove`::
 169
 170        Remove given amount of data from a given position of the buffer.
 171
 172`strbuf_splice`::
 173
 174        Remove the bytes between `pos..pos+len` and replace it with the given
 175        data.
 176
 177`strbuf_add_commented_lines`::
 178
 179        Add a NUL-terminated string to the buffer. Each line will be prepended
 180        by a comment character and a blank.
 181
 182`strbuf_add`::
 183
 184        Add data of given length to the buffer.
 185
 186`strbuf_addstr`::
 187
 188Add a NUL-terminated string to the buffer.
 189+
 190NOTE: This function will *always* be implemented as an inline or a macro
 191that expands to:
 192+
 193----
 194strbuf_add(..., s, strlen(s));
 195----
 196+
 197Meaning that this is efficient to write things like:
 198+
 199----
 200strbuf_addstr(sb, "immediate string");
 201----
 202
 203`strbuf_addbuf`::
 204
 205        Copy the contents of another buffer at the end of the current one.
 206
 207`strbuf_adddup`::
 208
 209        Copy part of the buffer from a given position till a given length to the
 210        end of the buffer.
 211
 212`strbuf_expand`::
 213
 214        This function can be used to expand a format string containing
 215        placeholders. To that end, it parses the string and calls the specified
 216        function for every percent sign found.
 217+
 218The callback function is given a pointer to the character after the `%`
 219and a pointer to the struct strbuf.  It is expected to add the expanded
 220version of the placeholder to the strbuf, e.g. to add a newline
 221character if the letter `n` appears after a `%`.  The function returns
 222the length of the placeholder recognized and `strbuf_expand()` skips
 223over it.
 224+
 225The format `%%` is automatically expanded to a single `%` as a quoting
 226mechanism; callers do not need to handle the `%` placeholder themselves,
 227and the callback function will not be invoked for this placeholder.
 228+
 229All other characters (non-percent and not skipped ones) are copied
 230verbatim to the strbuf.  If the callback returned zero, meaning that the
 231placeholder is unknown, then the percent sign is copied, too.
 232+
 233In order to facilitate caching and to make it possible to give
 234parameters to the callback, `strbuf_expand()` passes a context pointer,
 235which can be used by the programmer of the callback as she sees fit.
 236
 237`strbuf_expand_dict_cb`::
 238
 239        Used as callback for `strbuf_expand()`, expects an array of
 240        struct strbuf_expand_dict_entry as context, i.e. pairs of
 241        placeholder and replacement string.  The array needs to be
 242        terminated by an entry with placeholder set to NULL.
 243
 244`strbuf_addbuf_percentquote`::
 245
 246        Append the contents of one strbuf to another, quoting any
 247        percent signs ("%") into double-percents ("%%") in the
 248        destination. This is useful for literal data to be fed to either
 249        strbuf_expand or to the *printf family of functions.
 250
 251`strbuf_humanise_bytes`::
 252
 253        Append the given byte size as a human-readable string (i.e. 12.23 KiB,
 254        3.50 MiB).
 255
 256`strbuf_addf`::
 257
 258        Add a formatted string to the buffer.
 259
 260`strbuf_commented_addf`::
 261
 262        Add a formatted string prepended by a comment character and a
 263        blank to the buffer.
 264
 265`strbuf_fread`::
 266
 267        Read a given size of data from a FILE* pointer to the buffer.
 268+
 269NOTE: The buffer is rewound if the read fails. If -1 is returned,
 270`errno` must be consulted, like you would do for `read(3)`.
 271`strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the
 272same behaviour as well.
 273
 274`strbuf_read`::
 275
 276        Read the contents of a given file descriptor. The third argument can be
 277        used to give a hint about the file size, to avoid reallocs.
 278
 279`strbuf_read_file`::
 280
 281        Read the contents of a file, specified by its path. The third argument
 282        can be used to give a hint about the file size, to avoid reallocs.
 283
 284`strbuf_readlink`::
 285
 286        Read the target of a symbolic link, specified by its path.  The third
 287        argument can be used to give a hint about the size, to avoid reallocs.
 288
 289`strbuf_getline`::
 290
 291        Read a line from a FILE *, overwriting the existing contents
 292        of the strbuf. The second argument specifies the line
 293        terminator character, typically `'\n'`.
 294        Reading stops after the terminator or at EOF.  The terminator
 295        is removed from the buffer before returning.  Returns 0 unless
 296        there was nothing left before EOF, in which case it returns `EOF`.
 297
 298`strbuf_getwholeline`::
 299
 300        Like `strbuf_getline`, but keeps the trailing terminator (if
 301        any) in the buffer.
 302
 303`strbuf_getwholeline_fd`::
 304
 305        Like `strbuf_getwholeline`, but operates on a file descriptor.
 306        It reads one character at a time, so it is very slow.  Do not
 307        use it unless you need the correct position in the file
 308        descriptor.
 309
 310`stripspace`::
 311
 312        Strip whitespace from a buffer. The second parameter controls if
 313        comments are considered contents to be removed or not.
 314
 315`strbuf_split_buf`::
 316`strbuf_split_str`::
 317`strbuf_split_max`::
 318`strbuf_split`::
 319
 320        Split a string or strbuf into a list of strbufs at a specified
 321        terminator character.  The returned substrings include the
 322        terminator characters.  Some of these functions take a `max`
 323        parameter, which, if positive, limits the output to that
 324        number of substrings.
 325
 326`strbuf_list_free`::
 327
 328        Free a list of strbufs (for example, the return values of the
 329        `strbuf_split()` functions).
 330
 331`launch_editor`::
 332
 333        Launch the user preferred editor to edit a file and fill the buffer
 334        with the file's contents upon the user completing their editing. The
 335        third argument can be used to set the environment which the editor is
 336        run in. If the buffer is NULL the editor is launched as usual but the
 337        file's contents are not read into the buffer upon completion.