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

       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 speci-
       fied 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 ini-
               tial 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 ap-
       pend 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 posi-
       tion 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  er-
       ror.   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

           setbuf(stream, NULL);

       Upon successful completion, fmemopen() returns a FILE pointer.   Other-
       wise, 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 at-

       |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 longstand-
       ing bugs in the implementation of fmemopen() were fixed, and a new ver-
       sioned symbol was created for this interface.

   Binary mode
       From version 2.9 to 2.21, the glibc implementation of  fmemopen()  sup-
       ported  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 bi-
       nary mode, the 'b' must be the second character in mode.  Thus, for ex-
       ample,  "wb+"  has the desired effect, but "w+b" does not.  This is in-
       consistent 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, fmemo-
       pen() 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 implementa-
       tion 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 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 '<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 5.05 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

GNU                               2019-03-06                       FMEMOPEN(3)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2024 Hurricane Electric. All Rights Reserved.