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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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open_memstream(3) Library Functions Manual open_memstream(3)
open_memstream, open_wmemstream - open a dynamic memory buffer
stream
Standard C library (libc, -lc)
#include <stdio.h>
FILE *open_memstream(char **ptr, size_t *sizeloc);
#include <wchar.h>
FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
open_memstream(), open_wmemstream():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
The open_memstream() function opens a stream for writing to a
memory buffer. The function dynamically allocates the buffer, and
the buffer automatically grows as needed. Initially, the buffer
has a size of zero. After closing the stream, the caller should
free(3) this buffer.
The locations pointed to by ptr and sizeloc are used to report,
respectively, the current location and the size of the buffer.
The locations referred to by these pointers are updated each time
the stream is flushed (fflush(3)) and when the stream is closed
(fclose(3)). These values remain valid only as long as the caller
performs no further output on the stream. If further output is
performed, then the stream must again be flushed before trying to
access these values.
A null byte is maintained at the end of the buffer. This byte is
not included in the size value stored at sizeloc.
The stream maintains the notion of a current position, which is
initially zero (the start of the buffer). Each write operation
implicitly adjusts the buffer position. The stream's buffer
position can be explicitly changed with fseek(3) or fseeko(3).
Moving the buffer position past the end of the data already
written fills the intervening space with null characters.
The open_wmemstream() is similar to open_memstream(), but operates
on wide characters instead of bytes.
Upon successful completion, open_memstream() and open_wmemstream()
return a FILE pointer. Otherwise, NULL is returned and errno is
set to indicate the error.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ open_memstream(), open_wmemstream() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
open_memstream()
glibc 1.0.x.
open_wmemstream()
glibc 2.4.
There is no file descriptor associated with the file stream
returned by these functions (i.e., fileno(3) will return an error
if called on the returned stream).
Before glibc 2.7, seeking past the end of a stream created by
open_memstream() does not enlarge the buffer; instead the fseek(3)
call fails, returning -1.
See fmemopen(3).
fmemopen(3), fopen(3), setbuf(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 open_memstream(3)
Pages that refer to this page: fmemopen(3), fopen(3), malloc_info(3), stdio(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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