.\"	$OpenBSD: fmemopen.3,v 1.2 2013/01/03 08:05:40 jmc Exp $
.\"	$NetBSD: fmemopen.3,v 1.5 2010/10/07 00:14:14 enami Exp $
.\"
.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Christos Zoulas.
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.\"    contributors may be used to endorse or promote products derived
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.Dd $Mdocdate: January 3 2013 $
.Dt FMEMOPEN 3
.Os
.Sh NAME
.Nm fmemopen
.Nd open a stream that points to the given buffer
.Sh SYNOPSIS
.Fd #include <stdio.h>
.Ft FILE *
.Fn fmemopen "void *buffer" "size_t size" "const char *mode"
.Sh DESCRIPTION
The
.Fn fmemopen
function associates a stream with the given
.Fa buffer
and
.Fa size .
The
.Fa buffer
can be either
.Dv NULL ,
or must be of the given
.Fa size .
If the
.Fa buffer
is
.Dv NULL ,
a
.Fa buffer
of the given
.Fa size
will be dynamically allocated using
.Xr malloc 3
and released when
.Xr fclose 3
is called.
.Pp
The
.Fa mode
argument has the same meaning as in
.Xr fopen 3 .
.Pp
The stream treats the buffer as it would treat a file tracking the current
position to perform I/O operations.
For example, in the beginning the stream points to the beginning of the buffer,
unless
.Sq a
was specified in the
.Fa mode
argument, and then it points to the first
.Dv NUL
byte.
If a
.Dv NULL
.Fa buffer
was specified, then the stream will always point at the first byte of the
.Fa buffer .
.Pp
The stream also keeps track of the
.Fa size
of the
.Fa buffer .
The
.Fa size
is initialized depending on the mode:
.Bl -tag -width "r/w+XXX" -offset indent
.It Dv r/r+
Set to the
.Fa size
argument.
.It Dv w/w+
Set to
.Dv 0 .
.It Dv a/a+
Set to the first
.Dv NUL
byte, or the
.Fa size
argument if one is not found.
.El
.Pp
Read or write operations advance the buffer, but not to exceed the given
.Fa size
of the
.Fa buffer .
Trying to read beyond the
.Fa size
of the
.Fa buffer
results in
.Dv EOF
returned.
.Dv NUL
bytes are read normally.
Trying to write beyond the
.Fa size
of the
.Fa buffer
has no effect.
.Pp
When a stream open for writing is either flushed or closed, a
.Dv NUL
byte is written at the current position or at the end of the current
.Fa size
as kept internally.
.Sh RETURN VALUES
Upon successful completion,
.Fn fmemopen
returns a
.Dv FILE
pointer.
Otherwise,
.Dv NULL
is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa size
was
.Dv 0 ;
or the
.Fa mode
argument is invalid;
or the
.Fa buffer
argument is
.Dv NULL
and the
.Fa mode
argument does not specify a
.Sq + .
.El
.Pp
The
.Fn fmemopen
function
may also fail and set
.Va errno
for any of the errors
specified for the routine
.Xr malloc 3 .
.Sh SEE ALSO
.Xr fclose 3 ,
.Xr fflush 3 ,
.Xr fopen 3 ,
.Xr funopen 3 ,
.Xr malloc 3
.Sh STANDARDS
The function
.Fn fmemopen
conform to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn fmemopen
function first appeared in
.Ox 5.3 .