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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 the following call may be useful to detect errors at the time of an output operation:

           setbuf(stdream, NULL);

       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(stream, 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 indicate end-
       of-file only 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.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue   │
       │fopenmem(),       │ Thread safety │ MT-Safe │
       │open_memstream(), │               │         │
       │open_wmemstream   │               │         │


       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=%zu; ptr=%s\n", size, ptr);


       fopen(3), fopencookie(3)


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