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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FMEMOPEN(3P) POSIX Programmer's Manual FMEMOPEN(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.
fmemopen — open a memory buffer stream
#include <stdio.h>
FILE *fmemopen(void *restrict buf, size_t size,
const char *restrict mode);
The fmemopen() function shall associate the buffer given by the
buf and size arguments with a stream. The buf argument shall be
either a null pointer or point to a buffer that is at least size
bytes long.
The mode argument points to a string. If the string is one of the
following, the stream shall be opened in the indicated mode.
Otherwise, the behavior is undefined.
r Open the stream for reading.
w Open the stream for writing.
a Append; open the stream for writing at the first null
byte.
r+ Open the stream for update (reading and writing).
w+ Open the stream for update (reading and writing). Truncate
the buffer contents.
a+ Append; open the stream for update (reading and writing);
the initial position is at the first null byte.
Implementations shall accept all mode strings allowed by fopen(),
but the use of the character 'b' shall produce implementation-
defined results, where the resulting FILE * need not behave the
same as if 'b' were omitted.
If a null pointer is specified as the buf argument, fmemopen()
shall allocate size bytes of memory as if by a call to malloc().
This buffer shall be automatically freed when the stream is
closed. Because this feature is only useful when the stream is
opened for updating (because there is no way to get a pointer to
the buffer) the fmemopen() call may fail if the mode argument does
not include a '+'.
The stream shall maintain a current position in the buffer. This
position shall be initially set to either the beginning of the
buffer (for r and w modes) or to the first null byte in the buffer
(for a modes). If no null byte is found in append mode, the
initial position shall be set to one byte after the end of the
buffer.
If buf is a null pointer, the initial position shall always be set
to the beginning of the buffer.
The stream shall also maintain the size of the current buffer
contents; use of fseek() or fseeko() on the stream with SEEK_END
shall seek relative to this size. For modes r and r+ the size
shall be set to the value given by the size argument. For modes w
and w+ the initial size shall be zero and for modes a and a+ the
initial size shall be:
* Zero, if buf is a null pointer
* The position of the first null byte in the buffer, if one is
found
* The value of the size argument, if buf is not a null pointer
and no null byte is found
A read operation on the stream shall not advance the current
buffer position beyond the current buffer size. Reaching the
buffer size in a read operation shall count as ``end-of-file''.
Null bytes in the buffer shall have no special meaning for reads.
The read operation shall start at the current buffer position of
the stream.
A write operation shall start either at the current position of
the stream (if mode has not specified 'a' as the first character)
or at the current size of the stream (if mode had 'a' as the first
character). If the current position at the end of the write is
larger than the current buffer size, the current buffer size shall
be set to the current position. A write operation on the stream
shall not advance the current buffer size beyond the size given in
the size argument.
When a stream open for writing is flushed or closed, a null byte
shall be written at the current position or at the end of the
buffer, depending on the size of the contents. If a stream open
for update is flushed or closed and the last write has advanced
the current buffer size, a null byte shall be written at the end
of the buffer if it fits.
An attempt to seek a memory buffer stream to a negative position
or to a position larger than the buffer size given in the size
argument shall fail.
Upon successful completion, fmemopen() 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.
The fmemopen() function shall fail if:
EMFILE {STREAM_MAX} streams are currently open in the calling
process.
The fmemopen() function may fail if:
EINVAL The value of the mode argument is not valid.
EINVAL The buf argument is a null pointer and the mode argument
does not include a '+' character.
EINVAL The size argument specifies a buffer size of zero and the
implementation does not support this.
ENOMEM The buf argument is a null pointer and the allocation of a
buffer of length size has failed.
EMFILE {FOPEN_MAX} streams are currently open in the calling
process.
The following sections are informative.
#include <stdio.h>
#include <string.h>
static char buffer[] = "foobar";
int
main (void)
{
int ch;
FILE *stream;
stream = fmemopen(buffer, strlen (buffer), "r");
if (stream == NULL)
/* handle error */;
while ((ch = fgetc(stream)) != EOF)
printf("Got %c\n", ch);
fclose(stream);
return (0);
}
This program produces the following output:
Got f
Got o
Got o
Got b
Got a
Got r
None.
This interface has been introduced to eliminate many of the errors
encountered in the construction of strings, notably overflowing of
strings. This interface prevents overflow.
A future version of this standard may mandate specific behavior
when the mode argument includes 'b'.
A future version of this standard may require support of zero-
length buffer streams explicitly.
fdopen(3p), fopen(3p), freopen(3p), fseek(3p), malloc(3p),
open_memstream(3p)
The Base Definitions volume of POSIX.1‐2017, stdio.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 FMEMOPEN(3P)
Pages that refer to this page: stdio.h(0p), fclose(3p), fdopen(3p), fflush(3p), fopen(3p), freopen(3p), open_memstream(3p)
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HTML rendering created 2025-09-06 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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