#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 speci-
       fied 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 let-
       ter 'b' may be specified as the second character in  mode.   This  pro-
       vides  "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

       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  setbuf(fp, NULL)  may  be
       useful  to  detect errors at the time of an output operation.  Alterna-
       tively, the caller can explicitly set buf as the stdio  stream  buffer,
       at  the  same  time  informing  stdio  of the buffer's size, using set-
       buffer(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.

       When the stream is closed (fclose(3)) or flushed (fflush(3)), the loca-
       tions pointed to by ptr and sizeloc are  updated  to  contain,  respec-
       tively,  a  pointer  to  the buffer and the current size of the buffer.
       These values remain valid only as long as the caller performs  no  fur-
       ther  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_wmem-
       stream()  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.

       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.  Fur-
       thermore, 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  subse-

       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  pro-
       gram  scans  its  input string (taken from the program's first command-
       line argument) reading integers, and writes the squares of these  inte-
       gers  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)

       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

GNU                               2012-04-28                       FMEMOPEN(3)
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