open_memstream(3) Library Functions Manual open_memstream(3)
NAME
open_memstream, open_wmemstream - open a dynamic memory buffer stream
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
DESCRIPTION
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, respec-
tively, 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 out-
put 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 ini-
tially zero (the start of the buffer). Each write operation implicitly
adjusts the buffer position. The stream's buffer position can be ex-
plicitly changed with fseek(3) or fseeko(3). Moving the buffer posi-
tion 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.
RETURN VALUE
Upon successful completion, open_memstream() and open_wmemstream() re-
turn a FILE pointer. Otherwise, NULL is returned and errno is set to
indicate the error.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ open_memstream(), open_wmemstream() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2008.
HISTORY
open_memstream()
glibc 1.0.x.
open_wmemstream()
glibc 2.4.
NOTES
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).
BUGS
Before glibc 2.7, seeking past the end of a stream created by open_mem-
stream() does not enlarge the buffer; instead the fseek(3) call fails,
returning -1.
EXAMPLES
See fmemopen(3).
SEE ALSO
fmemopen(3), fopen(3), setbuf(3)
Linux man-pages 6.7 2023-12-28 open_memstream(3)
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