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.Dd August 1, 2015
.Dt OPEN_MEMSTREAM 3
.Os
.Sh NAME
.Nm open_memstream ,
.Nm open_wmemstream
.Nd dynamic memory buffer stream open functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft FILE *
.Fn open_memstream "char **bufp" "size_t *sizep"
.In wchar.h
.Ft FILE *
.Fn open_wmemstream "wchar_t **bufp" "size_t *sizep"
.Sh DESCRIPTION
The
.Fn open_memstream
and
.Fn open_wmemstream
functions create a write-only, seekable stream backed by a dynamically
allocated memory buffer.
The
.Fn open_memstream
function creates a byte-oriented stream,
while the
.Fn open_wmemstream
function creates a wide-oriented stream.
.Pp
Each stream maintains a current position and size.
Initially,
the position and size are set to zero.
Each write begins at the current position and advances it the number of
successfully written bytes for
.Fn open_memstream
or wide characters for
.Fn open_wmemstream .
If a write moves the current position beyond the length of the buffer,
the length of the buffer is extended and a null character is appended to the
buffer.
.Pp
A stream's buffer always contains a null character at the end of the buffer
that is not included in the current length.
.Pp
If a stream's current position is moved beyond the current length via a
seek operation and a write is performed,
the characters between the current length and the current position are filled
with null characters before the write is performed.
.Pp
After a successful call to
.Xr fclose 3
or
.Xr fflush 3 ,
the pointer referenced by
.Fa bufp
will contain the start of the memory buffer and the variable referenced by
.Fa sizep
will contain the smaller of the current position and the current buffer length.
.Pp
After a successful call to
.Xr fflush 3 ,
the pointer referenced by
.Fa bufp
and the variable referenced by
.Fa sizep
are only valid until the next write operation or a call to
.Xr fclose 3 .
.Pp
Once a stream is closed,
the allocated buffer referenced by
.Fa bufp
should be released via a call to
.Xr free 3
when it is no longer needed.
.Sh IMPLEMENTATION NOTES
Internally all I/O streams are effectively byte-oriented,
so using wide-oriented operations to write to a stream opened via
.Fn open_wmemstream
results in wide characters being expanded to a stream of multibyte characters
in stdio's internal buffers.
These multibyte characters are then converted back to wide characters when
written into the stream.
As a result,
the wide-oriented streams maintain an internal multibyte character conversion
state that is cleared on any seek operation that changes the current position.
This should have no effect as long as wide-oriented output operations are used
on a wide-oriented stream.
.Sh RETURN VALUES
Upon successful completion,
.Fn open_memstream
and
.Fn open_wmemstream
return a
.Vt 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 bufp
or
.Fa sizep
argument was
.Dv NULL .
.It Bq Er ENOMEM
Memory for the stream or buffer could not be allocated.
.El
.Sh SEE ALSO
.Xr fclose 3 ,
.Xr fflush 3 ,
.Xr fopen 3 ,
.Xr free 3 ,
.Xr fseek 3 ,
.Xr stdio 3 ,
.Xr sbuf 9
.Sh STANDARDS
The
.Fn open_memstream
and
.Fn open_wmemstream
functions conform to
.St -p1003.1-2008 .