OPEN_MEMSTREAM(3) Linux Programmer's Manual OPEN_MEMSTREAM(3)
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(7)):
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
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
explicitly 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.
Upon successful completion, open_memstream() and open_wmemstream()
return a FILE pointer. Otherwise, NULL is returned and errno is set to
indicate the error.
open_memstream() was already available in glibc 1.0.x. open_wmem-
stream() is available since glibc 2.4.
For an explanation of the terms used in this section, see
|Interface | Attribute | Value |
|open_memstream(), | Thread safety | MT-Safe |
|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 open_memstream() does not enlarge the buffer; instead the fseek(3)
call fails, returning -1.
fmemopen(3), fopen(3), setbuf(3)
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