open_memstream(3) — Linux manual page


OPEN_MEMSTREAM(3)         Linux Programmer's Manual        OPEN_MEMSTREAM(3)

NAME         top

       open_memstream,  open_wmemstream  -   open  a  dynamic  memory buffer

SYNOPSIS         top

       #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:

DESCRIPTION         top

       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.

RETURN VALUE         top

       Upon successful completion, open_memstream() and open_wmemstream()
       return a FILE pointer.  Otherwise, NULL is returned and errno is set
       to indicate the error.

VERSIONS         top

       open_memstream() was already available in glibc 1.0.x.
       open_wmemstream() is available since glibc 2.4.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface         Attribute     Value   │
       │open_memstream(), │ Thread safety │ MT-Safe │
       │open_wmemstream   │               │         │

CONFORMING TO         top

       POSIX.1-2008.  These functions are not specified in POSIX.1-2001, and
       are not widely available on other systems.

NOTES         top

       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         top

       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.

EXAMPLES         top

       See fmemopen(3).

SEE ALSO         top

       fmemopen(3), fopen(3), setbuf(3)

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

GNU                              2020-06-09                OPEN_MEMSTREAM(3)

Pages that refer to this page: fdopen(3)fmemopen(3)fopen(3)freopen(3)malloc_info(3)