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:

  1. The buf member is never NULL, so you 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).

  2. 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.

    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.

      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.

      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

This is 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.


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.

Add a single character to the buffer.


Insert data to the given position of the buffer. The remaining contents will be shifted, not overwritten.


Remove given amount of data from a given position of the buffer.


Remove the bytes between pos..pos+len and replace it with the given data.


Add data of given length to the buffer.


Add a NUL-terminated string to the buffer.

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");

Copy the contents of an other buffer at the end of the current one.


Copy part of the buffer from a given position till a given length to the end of the buffer.


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.

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.


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.


Add a formatted string to the buffer.


Read a given size of data from a FILE* pointer to the buffer.

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.

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.


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.


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.


Read a line from a FILE* pointer. The second argument specifies the line terminator character, typically '\n'.


Strip whitespace from a buffer. The second parameter controls if comments are considered contents to be removed or not.