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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
fmemopen — open a memory buffer stream
SYNOPSIS
#include <stdio.h>
FILE *fmemopen(void *restrict buf, size_t size,
const char *restrict mode);
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
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.
EXAMPLES
#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
APPLICATION USAGE
None.
RATIONALE
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.
FUTURE DIRECTIONS
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.
SEE ALSO
fdopen(), fopen(), freopen(), fseek(), malloc(), open_memstream()
The Base Definitions volume of POSIX.1‐2017, <stdio.h>
COPYRIGHT
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 .