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FMEMOPEN(3)                Linux Programmer's Manual               FMEMOPEN(3)

       fmemopen, open_memstream, open_wmemstream -  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);

       #define _GNU_SOURCE
       #include <wchar.h>

       FILE *open_wmemstream(wchar_t **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  posi-
       tion 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 tem-
       porary 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

       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 per-
       formed, 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

       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.   Other-
       wise, 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  sys-

       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() 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) read-
       ing 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 <assert.h>
       #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;

           assert(argc == 2);

           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=%ld; ptr=%s\n", (long) size, ptr);

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

GNU                               2009-04-21                       FMEMOPEN(3)