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       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

       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 buffer).

       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

       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 open_memstream(3)).

       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 this.

       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

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

       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 attributes(7).

       │InterfaceAttributeValue   │
       │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

   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 effect.


       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

           $ ./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)


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