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       fmemopen, open_memstream, open_wmemstream -  open memory as stream


       #define _GNU_SOURCE
       #include <stdio.h>

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

       FILE *open_memstream(char **ptr, size_t *sizeloc);

       #define _GNU_SOURCE
       #include <wchar.h>

       FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);


       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

       Attempts to write more than size bytes  to  the  buffer  result  in  an
       error.   (By  default,  such errors will only be visible 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

       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.


       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()
       call fails, returning -1.


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


       fopen(3), fopencookie(3), feature_test_macros(7)


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