open_memstream, open_wmemstream - open a dynamic memory buffer stream
FILE *open_memstream(char **ptr, size_t *sizeloc);
FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
Feature Test Macro Requirements for glibc (see feature_test_macros
- Since glibc 2.10:
- _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
() 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
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
(3)) and when the stream is closed (fclose
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
(3) or fseeko
(3). Moving the buffer position past the end
of the data already written fills the intervening space with null characters.
() 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
set to indicate the error.
() was already available in glibc 1.0.x.
() is available since glibc 2.4.
For an explanation of the terms used in this section, see attributes
|open_memstream (), open_wmemstream
POSIX.1-2008. These functions are not specified in POSIX.1-2001, and are not
widely available on other systems.
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
In glibc before version 2.7, seeking past the end of a stream created by
() does not enlarge the buffer; instead the
(3) call fails, returning -1.