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Diffstat (limited to 'Documentation/technical/api-strbuf.txt')
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diff --git a/Documentation/technical/api-strbuf.txt b/Documentation/technical/api-strbuf.txt new file mode 100644 index 0000000000..afe2759951 --- /dev/null +++ b/Documentation/technical/api-strbuf.txt @@ -0,0 +1,272 @@ +strbuf API +========== + +strbuf's are meant to be used with all the usual C string and memory +APIs. Given that the length of the buffer is known, it's often better to +use the mem* functions than a str* one (memchr vs. strchr e.g.). +Though, one has to be careful about the fact that str* functions often +stop on NULs and that strbufs may have embedded NULs. + +An strbuf is NUL terminated for convenience, but no function in the +strbuf API actually relies on the string being free of NULs. + +strbufs has some invariants that are very important to keep in mind: + +. The `buf` member is never NULL, so it can be used in any usual C +string operations safely. strbuf's _have_ to be initialized either by +`strbuf_init()` or by `= STRBUF_INIT` before the invariants, though. ++ +Do *not* assume anything on what `buf` really is (e.g. if it is +allocated memory or not), use `strbuf_detach()` to unwrap a memory +buffer from its strbuf shell in a safe way. That is the sole supported +way. This will give you a malloced buffer that you can later `free()`. ++ +However, it is totally safe to modify anything in the string pointed by +the `buf` member, between the indices `0` and `len-1` (inclusive). + +. The `buf` member is a byte array that has at least `len + 1` bytes + allocated. The extra byte is used to store a `'\0'`, allowing the + `buf` member to be a valid C-string. Every strbuf function ensure this + invariant is preserved. ++ +NOTE: It is OK to "play" with the buffer directly if you work it this + way: ++ +---- +strbuf_grow(sb, SOME_SIZE); <1> +strbuf_setlen(sb, sb->len + SOME_OTHER_SIZE); +---- +<1> Here, the memory array starting at `sb->buf`, and of length +`strbuf_avail(sb)` is all yours, and you can be sure that +`strbuf_avail(sb)` is at least `SOME_SIZE`. ++ +NOTE: `SOME_OTHER_SIZE` must be smaller or equal to `strbuf_avail(sb)`. ++ +Doing so is safe, though if it has to be done in many places, adding the +missing API to the strbuf module is the way to go. ++ +WARNING: Do _not_ assume that the area that is yours is of size `alloc +- 1` even if it's true in the current implementation. Alloc is somehow a +"private" member that should not be messed with. Use `strbuf_avail()` +instead. + +Data structures +--------------- + +* `struct strbuf` + +This is the string buffer structure. The `len` member can be used to +determine the current length of the string, and `buf` member provides access to +the string itself. + +Functions +--------- + +* Life cycle + +`strbuf_init`:: + + Initialize the structure. The second parameter can be zero or a bigger + number to allocate memory, in case you want to prevent further reallocs. + +`strbuf_release`:: + + Release a string buffer and the memory it used. You should not use the + string buffer after using this function, unless you initialize it again. + +`strbuf_detach`:: + + Detach the string from the strbuf and returns it; you now own the + storage the string occupies and it is your responsibility from then on + to release it with `free(3)` when you are done with it. + +`strbuf_attach`:: + + Attach a string to a buffer. You should specify the string to attach, + the current length of the string and the amount of allocated memory. + The amount must be larger than the string length, because the string you + pass is supposed to be a NUL-terminated string. This string _must_ be + malloc()ed, and after attaching, the pointer cannot be relied upon + anymore, and neither be free()d directly. + +`strbuf_swap`:: + + Swap the contents of two string buffers. + +* Related to the size of the buffer + +`strbuf_avail`:: + + Determine the amount of allocated but unused memory. + +`strbuf_grow`:: + + Ensure that at least this amount of unused memory is available after + `len`. This is used when you know a typical size for what you will add + and want to avoid repetitive automatic resizing of the underlying buffer. + This is never a needed operation, but can be critical for performance in + some cases. + +`strbuf_setlen`:: + + Set the length of the buffer to a given value. This function does *not* + allocate new memory, so you should not perform a `strbuf_setlen()` to a + length that is larger than `len + strbuf_avail()`. `strbuf_setlen()` is + just meant as a 'please fix invariants from this strbuf I just messed + with'. + +`strbuf_reset`:: + + Empty the buffer by setting the size of it to zero. + +* Related to the contents of the buffer + +`strbuf_rtrim`:: + + Strip whitespace from the end of a string. + +`strbuf_cmp`:: + + Compare two buffers. Returns an integer less than, equal to, or greater + than zero if the first buffer is found, respectively, to be less than, + to match, or be greater than the second buffer. + +* Adding data to the buffer + +NOTE: All of the functions in this section will grow the buffer as necessary. +If they fail for some reason other than memory shortage and the buffer hadn't +been allocated before (i.e. the `struct strbuf` was set to `STRBUF_INIT`), +then they will free() it. + +`strbuf_addch`:: + + Add a single character to the buffer. + +`strbuf_insert`:: + + Insert data to the given position of the buffer. The remaining contents + will be shifted, not overwritten. + +`strbuf_remove`:: + + Remove given amount of data from a given position of the buffer. + +`strbuf_splice`:: + + Remove the bytes between `pos..pos+len` and replace it with the given + data. + +`strbuf_add`:: + + Add data of given length to the buffer. + +`strbuf_addstr`:: + +Add a NUL-terminated string to the buffer. ++ +NOTE: This function will *always* be implemented as an inline or a macro +that expands to: ++ +---- +strbuf_add(..., s, strlen(s)); +---- ++ +Meaning that this is efficient to write things like: ++ +---- +strbuf_addstr(sb, "immediate string"); +---- + +`strbuf_addbuf`:: + + Copy the contents of an other buffer at the end of the current one. + +`strbuf_adddup`:: + + Copy part of the buffer from a given position till a given length to the + end of the buffer. + +`strbuf_expand`:: + + This function can be used to expand a format string containing + placeholders. To that end, it parses the string and calls the specified + function for every percent sign found. ++ +The callback function is given a pointer to the character after the `%` +and a pointer to the struct strbuf. It is expected to add the expanded +version of the placeholder to the strbuf, e.g. to add a newline +character if the letter `n` appears after a `%`. The function returns +the length of the placeholder recognized and `strbuf_expand()` skips +over it. ++ +The format `%%` is automatically expanded to a single `%` as a quoting +mechanism; callers do not need to handle the `%` placeholder themselves, +and the callback function will not be invoked for this placeholder. ++ +All other characters (non-percent and not skipped ones) are copied +verbatim to the strbuf. If the callback returned zero, meaning that the +placeholder is unknown, then the percent sign is copied, too. ++ +In order to facilitate caching and to make it possible to give +parameters to the callback, `strbuf_expand()` passes a context pointer, +which can be used by the programmer of the callback as she sees fit. + +`strbuf_expand_dict_cb`:: + + Used as callback for `strbuf_expand()`, expects an array of + struct strbuf_expand_dict_entry as context, i.e. pairs of + placeholder and replacement string. The array needs to be + terminated by an entry with placeholder set to NULL. + +`strbuf_addbuf_percentquote`:: + + Append the contents of one strbuf to another, quoting any + percent signs ("%") into double-percents ("%%") in the + destination. This is useful for literal data to be fed to either + strbuf_expand or to the *printf family of functions. + +`strbuf_addf`:: + + Add a formatted string to the buffer. + +`strbuf_fread`:: + + Read a given size of data from a FILE* pointer to the buffer. ++ +NOTE: The buffer is rewound if the read fails. If -1 is returned, +`errno` must be consulted, like you would do for `read(3)`. +`strbuf_read()`, `strbuf_read_file()` and `strbuf_getline()` has the +same behaviour as well. + +`strbuf_read`:: + + Read the contents of a given file descriptor. The third argument can be + used to give a hint about the file size, to avoid reallocs. + +`strbuf_read_file`:: + + Read the contents of a file, specified by its path. The third argument + can be used to give a hint about the file size, to avoid reallocs. + +`strbuf_readlink`:: + + Read the target of a symbolic link, specified by its path. The third + argument can be used to give a hint about the size, to avoid reallocs. + +`strbuf_getline`:: + + Read a line from a FILE* pointer. The second argument specifies the line + terminator character, typically `'\n'`. + +`stripspace`:: + + Strip whitespace from a buffer. The second parameter controls if + comments are considered contents to be removed or not. + +`launch_editor`:: + + Launch the user preferred editor to edit a file and fill the buffer + with the file's contents upon the user completing their editing. The + third argument can be used to set the environment which the editor is + run in. If the buffer is NULL the editor is launched as usual but the + file's contents are not read into the buffer upon completion. |