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OPEN_MEMSTREAM(3P) POSIX Programmer's Manual OPEN_MEMSTREAM(3P)
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.
open_memstream, open_wmemstream — open a dynamic memory buffer stream
#include <stdio.h>
FILE *open_memstream(char **bufp, size_t *sizep);
#include <wchar.h>
FILE *open_wmemstream(wchar_t **bufp, size_t *sizep);
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
successfully 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 position. In this case a null character for
open_memstream() or a null wide character for open_wmemstream() shall
be appended to the current buffer. 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 current 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().
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.
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.
#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
The buffer created by these functions should be freed by the
application after closing the stream, by means of a call to free().
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.
None.
fclose(3p), fdopen(3p), fflush(3p), fmemopen(3p), fopen(3p),
free(3p), freopen(3p)
The Base Definitions volume of POSIX.1‐2008, stdio.h(0p), wchar.h(0p)
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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 OPEN_MEMSTREAM(3P)
Pages that refer to this page: stdio.h(0p), wchar.h(0p), fclose(3p), fdopen(3p), fflush(3p), fmemopen(3p), fopen(3p), freopen(3p)