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

       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(), open_wmemstream()   │ Thread safety │ MT-Safe │

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 5.13 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                            2021-03-22              OPEN_MEMSTREAM(3)

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