fmemopen -  open memory as stream


   #include <stdio.h>

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       Since glibc 2.10:
           _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.

   The  mode argument specifies the semantics of I/O on the stream, and is
   one of the following:

   r       The stream is opened for reading.

   w       The stream is opened for writing.

   a       Append; open the stream for writing, with  the  initial  buffer
           position set to the first null byte.

   r+      Open the stream for reading and writing.

   w+      Open  the  stream for reading and writing.  The buffer contents
           are truncated (i.e., '\0' is placed in the first  byte  of  the

   a+      Append;  open  the  stream  for  reading  and writing, with the
           initial buffer position set to the first null byte.

   The stream maintains the notion of a  current  position,  the  location
   where  the  next I/O operation will be performed.  The current position
   is implicitly updated by I/O operations.  It can be explicitly  updated
   using fseek(3), and determined using ftell(3).  In all modes other than
   append, the initial position is set to the start  of  the  buffer.   In
   append  mode,  if  no  null  byte  is found within the buffer, then the
   initial position is size+1.

   If buf is specified as NULL, then fmemopen() allocates a buffer of size
   bytes.  This is useful for an application that wants to write data to a
   temporary buffer and then read it back again.  The initial position  is
   set to the start of the buffer.  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

   If buf is not NULL, then it should point to a buffer of  at  least  len
   bytes allocated by the caller.

   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

   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 current buffer
   position advances size bytes past the start of the buffer.

   Write operations take place either at the current position  (for  modes
   other  than  append),  or at the current size of the stream (for append

   Attempts to write more than size bytes  to  the  buffer  result  in  an
   error.   By  default,  such  errors  will be visible (by the absence of
   data) 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(stream, NULL);


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


   fmemopen() was already available in glibc 1.0.x.


   For   an   explanation   of   the  terms  used  in  this  section,  see

   Interface    Attribute      Value   
   fmemopen(),  Thread safety  MT-Safe 


   POSIX.1-2008.  This function is not specified in POSIX.1-2001,  and  is
   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
   this  function  (i.e.,  fileno(3) will return an error if called on the
   returned stream).

   With  version  2.22,  binary  mode  (see  below)  was   removed,   many
   longstanding bugs in the implementation of fmemopen() were fixed, and a
   new versioned symbol was created for this interface.

   Binary mode
   From version 2.9  to  2.21,  the  glibc  implementation  of  fmemopen()
   supported  a "binary" mode, enabled by specifying the letter 'b' as the
   second character in mode.  In this 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.

   An  API  bug  afflicted  the  implementation of binary mode: to specify
   binary mode, 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).

   Binary mode was removed in glibc 2.22; a 'b' specified in mode  has  no


   In  versions  of  glibc  before  2.22,  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;  since  version  2.22,  the  glibc
   implementation provides that behavior.

   In  versions of glibc before 2.22, specifying append mode ("a" or "a+")
   for fmemopen() sets the initial buffer position to the first null byte,
   but  (if the current position 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.  This bug is fixed in glibc 2.22.

   In  versions  of  glibc before 2.22, 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 buffer
   position should be set to the next byte after the end  of  the  buffer.
   However,  in this case the glibc fmemopen() sets the buffer position to
   -1.  This bug is fixed in glibc 2.22.

   In versions of glibc before 2.22, when a call to fseek(3) with a whence
   value  of SEEK_END was performed on a stream created by fmemopen(), the
   offset was subtracted from the end-of-stream position, instead of being
   added.  This bug is fixed in glibc 2.22.

   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(3)  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 '<num>...'\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);



   fopen(3), fopencookie(3), open_memstream(3)


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at


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