NAME
fmemopen, open_memstream, open_wmemstream - open memory as stream
SYNOPSIS
#define _GNU_SOURCE
#include <stdio.h>
FILE *fmemopen(void *buf, size_t size, const char *mode);
FILE *open_memstream(char **ptr, size_t *sizeloc);
#define _GNU_SOURCE
#include <wchar.h>
FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
DESCRIPTION
The fmemopen() function opens a stream that permits the access
specified by mode. The stream allows I/O to be performed on the string
or memory buffer pointed to by buf. This buffer must be at least size
bytes long.
The argument mode is the same as for fopen(3). If mode specifies an
append mode, then the initial file position is set to the location of
the first null byte ('\0') in the buffer; otherwise the initial file
position is set to the start of the buffer. Since glibc 2.9, the
letter ’b’ may be specified as the second character in mode. This
provides "binary" mode: writes don’t implicitly add a terminating null
byte, and fseek(3) SEEK_END is relative to the end of the buffer (i.e.,
the value specified by the size argument), rather than the current
string length.
When a stream that has been opened for writing is flushed (fflush(3))
or closed (fclose(3)), a null byte is written at the end of the buffer
if there is space. The caller should ensure that an extra byte is
available in the buffer (and that size counts that byte) to allow for
this.
Attempts to write more than size bytes to the buffer result in an
error. (By default, such errors will only be visible when the stdio
buffer is flushed. Disabling buffering with setbuf(fp, NULL) may be
useful to detect errors at the time of an output operation.
Alternatively, the caller can explicitly set buf as the stdio stream
buffer, at the same time informing stdio of the buffer’s size, using
setbuffer(fp, buf, size).)
In a stream opened for reading, null bytes ('\0') in the buffer do not
cause read operations to return an end-of-file indication. A read from
the buffer will only indicate end-of-file when the file pointer
advances size bytes past the start of the buffer.
If buf is specified as NULL, then fmemopen() dynamically allocates a
buffer size bytes long. This is useful for an application that wants
to write data to a temporary buffer and then read it back again. The
buffer is automatically freed when the stream is closed. Note that the
caller has no way to obtain a pointer to the temporary buffer allocated
by this call (but see open_memstream() below).
The open_memstream() function opens a stream for writing to a buffer.
The buffer is dynamically allocated (as with malloc(3)), and
automatically grows as required. After closing the stream, the caller
should free(3) this buffer.
When the stream is closed (fclose(3)) or flushed (fflush(3)), the
locations pointed to by ptr and sizeloc are updated to contain,
respectively, a pointer to the buffer and the current size of the
buffer. These values remain valid only as long as the caller performs
no further output on the stream. If further output is performed, then
the stream must again be flushed before trying to access these
variables.
A null byte is maintained at the end of the buffer. This byte is not
included in the size value stored at sizeloc.
The stream’s file position can be changed with fseek(3) or fseeko(3).
Moving the file position past the end of the data already written fills
the intervening space with zeros.
The open_wmemstream() is similar to open_memstream(), but operates on
wide characters instead of bytes.
RETURN VALUE
Upon successful completion fmemopen(), open_memstream() and
open_wmemstream() return a FILE pointer. Otherwise, NULL is returned
and errno is set to indicate the error.
VERSIONS
fmemopen() and open_memstream() were already available in glibc 1.0.x.
open_wmemstream() is available since glibc 2.4.
CONFORMING TO
POSIX.1-2008. These functions are not specified in POSIX.1-2001, and
are not widely available on other systems.
NOTES
There is no file descriptor associated with the file stream returned by
these functions (i.e., fileno(3) will return an error if called on the
returned stream).
BUGS
In glibc before version 2.7, seeking past the end of a stream created
by open_memstream() does not enlarge the buffer; instead the fseek()
call fails, returning -1.
EXAMPLE
The program below uses fmemopen() to open an input buffer, and
open_memstream() to open a dynamically sized output buffer. The
program scans its input string (taken from the program’s first command-
line argument) reading integers, and writes the squares of these
integers to the output buffer. An example of the output produced by
this program is the following:
$ ./a.out '1 23 43'
size=11; ptr=1 529 1849
Program source
#define _GNU_SOURCE
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
FILE *out, *in;
int v, s;
size_t size;
char *ptr;
if (argc != 2) {
fprintf(stderr, "Usage: %s <file>\n", argv[0]);
exit(EXIT_FAILURE);
}
in = fmemopen(argv[1], strlen(argv[1]), "r");
if (in == NULL)
handle_error("fmemopen");
out = open_memstream(&ptr, &size);
if (out == NULL)
handle_error("fmemopen");
for (;;) {
s = fscanf(in, "%d", &v);
if (s <= 0)
break;
s = fprintf(out, "%d ", v * v);
if (s == -1)
handle_error("fprintf");
}
fclose(in);
fclose(out);
printf("size=%ld; ptr=%s\n", (long) size, ptr);
free(ptr);
exit(EXIT_SUCCESS);
}
SEE ALSO
fopen(3), fopencookie(3), feature_test_macros(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.