NAME

       open_memstream, open_wmemstream -  open a dynamic memory buffer stream

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, 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

       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

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

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────┬───────────────┬─────────┐
       │Interface         │ Attribute     │ Value   │
       ├──────────────────┼───────────────┼─────────┤
       │open_memstream(), │ Thread safety │ MT-Safe │
       │open_wmemstream   │               │         │
       └──────────────────┴───────────────┴─────────┘

CONFORMING TO

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

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

       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.

EXAMPLE

       See fmemopen(3).

SEE ALSO

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

COLOPHON

       This  page  is  part  of  release  5.05  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
       https://www.kernel.org/doc/man-pages/.