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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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().
After a successful fclose(), the pointer referenced by bufp can
be passed to free().
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 shall fail if:
EMFILE {STREAM_MAX} streams are currently open in the calling
process.
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.1‐2017, stdio.h(0p),
wchar.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/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 2017 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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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