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       fmemopen, open_memstream, open_wmemstream -  open memory as stream


       #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 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 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 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  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.
       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 temporary 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 buffer.

       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 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_wmemstream() 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.  Furthermore, 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 subsequent writes to append at the end of the stream.

       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  file
       position  should  be  set  to the next byte after the end of the buffer.  However, in this
       case the glibc fmemopen() sets the file position to -1.

       To specify binary mode for fmemopen() 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).

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


       fopen(3), fopencookie(3)


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