.\"	$NetBSD: fmemopen.3,v 1.9 2015/09/06 03:10:50 pgoyette Exp $
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.\" by Christos Zoulas.
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.Dd September 5, 2015
.Dt FMEMOPEN 3
.Os
.Sh NAME
.Nm fmemopen
.Nd open a stream that points to a memory buffer
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft FILE *
.Fn fmemopen "void *restrict buffer" "size_t size" "const char *restrict 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 freed 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
.Dv 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+
.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, if there is room.
.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
.Dv + .
.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 malloc 3
.Sh STANDARDS
The
.Fn fmemopen
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn fmemopen
function first appeared in
.Nx 6.0 .