fmemopen(3) open memory as stream

Other Alias

open_memstream, open_wmemstream


#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:


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 be visible only when the stdio buffer is flushed. Disabling buffering with the following call may be useful to detect errors at the time of an output operation:

    setbuf(stdream, NULL);

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(stream, 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 indicate end-of-file only 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.


For an explanation of the terms used in this section, see attributes(7).
Thread safetyMT-Safe


POSIX.1-2008. These functions are not specified in POSIX.1-2001, and are not widely available on other systems.

POSIX.1-2008 specifies that 'b' in mode shall be ignored. However, Technical Corrigendum 1 adjusts the standard to allow implementation-specific treatment for this case, thus permitting the glibc treatment of 'b'.


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.

If size is specified as zero, fmemopen() fails with the error EINVAL. It would be more consistent if this case successfully created a stream that then returned end of file on the first attempt at reading. Furthermore, POSIX.1-2008 does not specify a failure for this case.

Specifying append mode ("a" or "a+") for fmemopen() sets the initial file position to the first null byte, but (if the file offset is reset to a location other than the end of the stream) does not force subsequent writes to append at the end of the stream.

If the mode argument to fmemopen() specifies append ("a" or "a+"), and the size argument does not cover a null byte in buf, then, according to POSIX.1-2008, the initial file position should be set to the next byte after the end of the buffer. However, in this case the glibc fmemopen() sets the file position to -1.

To specify binary mode for fmemopen() the 'b' must be the second character in mode. Thus, for example, "wb+" has the desired effect, but "w+b" does not. This is inconsistent with the treatment of mode by fopen(3).

The glibc 2.9 addition of "binary" mode for fmemopen() silently changed the ABI: previously, fmemopen() ignored 'b' in mode.


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)
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]);
    in = fmemopen(argv[1], strlen(argv[1]), "r");
    if (in == NULL)
    out = open_memstream(&ptr, &size);
    if (out == NULL)
    for (;;) {
        s = fscanf(in, "%d", &v);
        if (s <= 0)
        s = fprintf(out, "%d ", v * v);
        if (s == -1)
    printf("size=%zu; ptr=%s\n", size, ptr);


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