Section: glibc function (3)
Updated: 2005-12-08
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fmemopen, open_memstream - open memory as stream  


#define _GNU_SOURCE
#include <stdio.h>

FILE *fmemopen(void *buf, size_t size, const char *mode);

FILE *open_memstream(char ** ptr, size_t *sizeloc)  


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 location of the first null byte ('\0') in the buffer; otherwise the initial file position is set to the start of the buffer.

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() 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.  


Upon successful completion fmemopen() and open_memstream() return a FILE pointer. Otherwise, NULL is returned and the global variable errno is set to indicate the error.



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 <assert.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
    FILE *out, *in;
    int v, s;
    size_t size;
    char *ptr;

    assert(argc == 2);

    in = fmemopen(argv[1], strlen(argv[1]), "r");
    if (in == NULL) { perror("fmemopen"); exit(EXIT_FAILURE);}

    out = open_memstream(&ptr, &size);
    if (out == NULL) { perror("fmemopen"); exit(EXIT_FAILURE);}

    for (;;) {
        s = fscanf(in, "%d", &v);
        if (s <= 0)

        s = fprintf(out, "%d ", v * v);
        if (s == -1) { perror("fprintf"); exit(EXIT_FAILURE); }
    printf("size=%ld; ptr=%s\n", (long) size, ptr);



These functions are GNU extensions.  





SEE ALSO manual pages