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:

#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);
}

SEE ALSO

fopen(3), fopencookie(3), feature_test_macros(7)