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FMEMOPEN(3) Linux Programmer's Manual FMEMOPEN(3)
fmemopen, open_memstream, open_wmemstream - open memory as stream
#include <stdio.h>
FILE *fmemopen(void *buf, size_t size, const char *mode);
FILE *open_memstream(char **ptr, size_t *sizeloc);
#include <wchar.h>
FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fmemopen(), open_memstream(), open_wmemstream():
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
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.
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.
fmemopen() and open_memstream() were already available in glibc 1.0.x.
open_wmemstream() is available since glibc 2.4.
POSIX.1-2008. These functions are not specified in POSIX.1-2001, and are not
widely available on other systems.
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).
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(3) call fails,
returning -1.
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
#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("open_memstream");
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);
}
fopen(3), fopencookie(3)
This page is part of release 3.32 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/.
GNU 2010-09-15 FMEMOPEN(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface