man-pages/man3/open_memstream.3

.\" Copyright 2005 walter harms (walter.harms@informatik.uni-oldenburg.de),
.\" and Copyright 2005, 2012, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
.\" Distributed under the GPL.
.\" %%%LICENSE_END
.\"
.\" 2008-12-04, Petr Baudis <pasky@suse.cz>: Document open_wmemstream()
.\"
.TH OPEN_MEMSTREAM 3 2015-03-29 "GNU" "Linux Programmer's Manual"
.SH NAME
open_memstream, open_wmemstream \-  open a dynamic memory buffer stream
.SH SYNOPSIS
.nf
.B #include <stdio.h>

.BI "FILE *open_memstream(char **" ptr ", size_t *" sizeloc );

.B #include <wchar.h>

.BI "FILE *open_wmemstream(wchar_t **" ptr ", size_t *" sizeloc );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR open_memstream (),
.BR open_wmemstream ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.10:
_POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_GNU_SOURCE
.RE
.ad
.PD
.SH DESCRIPTION
The
.BR open_memstream ()
function opens a stream for writing to a buffer.
The function dynamically allocates the buffer (in the manner of
.BR malloc (3)),
and the buffer automatically grows as required.
After closing the stream, the caller should
.BR free (3)
this buffer.

The locations pointed to by
.IR ptr
and
.I sizeloc
are used to report the current location and size of the buffer.
When the stream is closed
.RB ( fclose (3))
or flushed
.RB ( fflush (3)),
the locations pointed to by
.I ptr
and
.I sizeloc
are updated to contain, respectively, a pointer to the buffer and the
current size of the buffer.
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 variables.

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 .

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
zeros.

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 VERSIONS
.BR open_memstream ()
was already available in glibc 1.0.x.
.BR open_wmemstream ()
is available since glibc 2.4.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR open_memstream (),
.br
.BR open_wmemstream
T}	Thread safety	MT-Safe
.TE

.SH CONFORMING TO
POSIX.1-2008.
These functions are not specified in POSIX.1-2001,
and are not widely available on other systems.
.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
In glibc before version 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 EXAMPLE
See
.BR fmemopen (3).
.SH SEE ALSO
.BR fmemopen (3),
.BR fopen (3)