'\" t
.\" Copyright 2005, 2012, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: GPL-1.0-or-later
.\"
.\" 2008-12-04, Petr Baudis <pasky@suse.cz>: Document open_wmemstream()
.\"
.TH open_memstream 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
open_memstream, open_wmemstream \-  open a dynamic memory buffer stream
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "FILE *open_memstream(char **" ptr ", size_t *" sizeloc );
.PP
.B #include <wchar.h>
.PP
.BI "FILE *open_wmemstream(wchar_t **" ptr ", size_t *" sizeloc );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR open_memstream (),
.BR open_wmemstream ():
.nf
    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _GNU_SOURCE
.fi
.SH DESCRIPTION
The
.BR 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
.BR free (3)
this buffer.
.PP
The locations pointed to by
.I ptr
and
.I 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
.RB ( fflush (3))
and when the stream is closed
.RB ( 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.
.PP
A null byte is maintained at the end of the buffer.
This byte is
.I not
included in the size value stored at
.IR sizeloc .
.PP
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
.BR fseek (3)
or
.BR fseeko (3).
Moving the buffer position past the end
of the data already written fills the intervening space with
null characters.
.PP
The
.BR open_wmemstream ()
is similar to
.BR open_memstream (),
but operates on wide characters instead of bytes.
.SH RETURN VALUE
Upon successful completion,
.BR open_memstream ()
and
.BR open_wmemstream ()
return a
.I FILE
pointer.
Otherwise, NULL is returned and
.I errno
is set to indicate the error.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR open_memstream (),
.BR open_wmemstream ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
.TP
.BR open_memstream ()
glibc 1.0.x.
.TP
.BR open_wmemstream ()
glibc 2.4.
.SH NOTES
There is no file descriptor associated with the file stream
returned by these functions
(i.e.,
.BR fileno (3)
will return an error if called on the returned stream).
.SH BUGS
Before glibc 2.7, seeking past the end of a stream created by
.BR open_memstream ()
does not enlarge the buffer; instead the
.BR fseek (3)
call fails, returning \-1.
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=1996
.SH EXAMPLES
See
.BR fmemopen (3).
.SH SEE ALSO
.BR fmemopen (3),
.BR fopen (3),
.BR setbuf (3)