OPEN_MEMSTREAM(3P) POSIX Programmer's Manual OPEN_MEMSTREAM(3P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
open_memstream, open_wmemstream — open a dynamic memory buffer stream
SYNOPSIS
#include <stdio.h>
FILE *open_memstream(char **bufp, size_t *sizep);
#include <wchar.h>
FILE *open_wmemstream(wchar_t **bufp, size_t *sizep);
DESCRIPTION
The open_memstream() and open_wmemstream() functions shall create an
I/O stream associated with a dynamically allocated memory buffer. The
stream shall be opened for writing and shall be seekable.
The stream associated with a call to open_memstream() shall be byte-
oriented.
The stream associated with a call to open_wmemstream() shall be wide-
oriented.
The stream shall maintain a current position in the allocated buffer
and a current buffer length. The position shall be initially set to
zero (the start of the buffer). Each write to the stream shall start at
the current position and move this position by the number of success‐
fully written bytes for open_memstream() or the number of successfully
written wide characters for open_wmemstream(). The length shall be
initially set to zero. If a write moves the position to a value larger
than the current length, the current length shall be set to this posi‐
tion. In this case a null character for open_memstream() or a null wide
character for open_wmemstream() shall be appended to the current buf‐
fer. For both functions the terminating null is not included in the
calculation of the buffer length.
After a successful fflush() or fclose(), the pointer referenced by bufp
shall contain the address of the buffer, and the variable pointed to by
sizep shall contain the smaller of the current buffer length and the
number of bytes for open_memstream(), or the number of wide characters
for open_wmemstream(), between the beginning of the buffer and the cur‐
rent file position indicator.
After a successful fflush() the pointer referenced by bufp and the
variable referenced by sizep remain valid only until the next write
operation on the stream or a call to fclose().
RETURN VALUE
Upon successful completion, these functions shall return a pointer to
the object controlling the stream. Otherwise, a null pointer shall be
returned, and errno shall be set to indicate the error.
ERRORS
These functions may fail if:
EINVAL bufp or sizep are NULL.
EMFILE {FOPEN_MAX} streams are currently open in the calling process.
ENOMEM Memory for the stream or the buffer could not be allocated.
The following sections are informative.
EXAMPLES
#include <stdio.h>
#include <stdlib.h>
int
main (void)
{
FILE *stream;
char *buf;
size_t len;
off_t eob;
stream = open_memstream (&buf, &len);
if (stream == NULL)
/* handle error */ ;
fprintf (stream, "hello my world");
fflush (stream);
printf ("buf=%s, len=%zu\n", buf, len);
eob = ftello(stream);
fseeko (stream, 0, SEEK_SET);
fprintf (stream, "good-bye");
fseeko (stream, eob, SEEK_SET);
fclose (stream);
printf ("buf=%s, len=%zu\n", buf, len);
free (buf);
return 0;
}
This program produces the following output:
buf=hello my world, len=14
buf=good-bye world, len=14
APPLICATION USAGE
The buffer created by these functions should be freed by the applica‐
tion after closing the stream, by means of a call to free().
RATIONALE
These functions are similar to fmemopen() except that the memory is
always allocated dynamically by the function, and the stream is opened
only for output.
FUTURE DIRECTIONS
None.
SEE ALSOfclose(), fdopen(), fflush(), fmemopen(), fopen(), free(), freopen()
The Base Definitions volume of POSIX.1‐2008, <stdio.h>, <wchar.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 OPEN_MEMSTREAM(3P)
