fmemopen / open_memstream / fopencookie (3) - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <stdio.h>

FILE *fopencookie(void *restrict cookie, const char *restrict mode,
                  cookie_io_functions_t io_funcs);        /* __USE_MISC */

FILE *fmemopen(void *s, size_t len, const char *modes);   /* XOPEN2K8 */

FILE *open_memstream(char **bufloc, size_t *sizeloc);     /* XOPEN2K8 */

fopencookie is in the __USE_MISC block of stdio.h (lines 303-310) – the man page asks for _GNU_SOURCE, which implies it – fmemopen/open_memstream in the #if defined __USE_XOPEN2K8 || __GLIBC_USE (LIB_EXT2) block (lines 312-322).

Semantics

fmemopen opens a stream on the caller’s buffer of len bytes: mode a/a+ starts the position at the first null byte (len+1 if none), a null byte is written at the end on flush/close if space remains, and reads do not stop at embedded nulls. With s == NULL glibc allocates len bytes, freed on fclose, with no way to retrieve the pointer (fmemopen(3), manual String Streams). open_memstream allocates a growing buffer and publishes *bufloc/*sizeloc on every fflush or fclose; the values stay valid only until the next output, the trailing null is not counted in the size, and seeking past the end zero-fills. fopencookie builds a stream over four caller hook functions (read, write, seek, close in cookie_io_functions_t) receiving the opaque cookie as first argument.

Gotchas and portability

• fileno(3) fails on all three (no descriptor behind the stream); freopen(3) also fails on them (manual, Opening Streams).
• glibc 2.22 split: binary mode (b) removed and several append/ SEEK_END/zero-size bugs fixed behind the new fmemopen@@GLIBC_2.22 symbol; old binaries keep the buggy @GLIBC_2.2.5 behavior (fmemopen(3) NOTES/BUGS).
• Pre-2.22, fmemopen(..., 0, ...) failed with EINVAL.
• musl 1.2.5: all three present (musl src/stdio/fmemopen.c, open_memstream.c, fopencookie.c).

References

• glibc manual: stdio.texi, nodes String Streams, Custom Streams, Streams and Cookies
• man3: fmemopen(3), open_memstream(3), fopencookie(3)
• headers: /usr/include/stdio.h
• symbols: nm -D --with-symbol-versions libc.so.6