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OPEN_MEMSTREAM(3P)         POSIX Programmer's Manual        OPEN_MEMSTREAM(3P)
PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       open_memstream, open_wmemstream -- open a dynamic memory buffer stream
SYNOPSIS
       #include <stdio.h>
       FILE *open_memstream(char **bufp, size_t *sizep);
       #include <wchar.h>
       FILE *open_wmemstream(wchar_t **bufp, size_t *sizep);
DESCRIPTION
       The open_memstream() and open_wmemstream() functions  shall  create  an
       I/O  stream  associated with a dynamically allocated memory buffer. The
       stream shall be opened for writing and shall be seekable.
       The stream associated with a call to open_memstream()  shall  be  byte-
       oriented.
       The  stream  associated with a call to open_wmemstream() shall be wide-
       oriented.
       The stream shall maintain a current position in  the  allocated  buffer
       and  a  current  buffer  length. The position shall be initially set to
       zero (the start of the buffer). Each write to the stream shall start at
       the  current  position and move this position by the number of success-
       fully written bytes for open_memstream() or the number of  successfully
       written  wide  characters  for  open_wmemstream().  The length shall be
       initially set to zero. If a write moves the position to a value  larger
       than  the current length, the current length shall be set to this posi-
       tion. In this case a null character for open_memstream() or a null wide
       character  for  open_wmemstream() shall be appended to the current buf-
       fer. For both functions the terminating null is  not  included  in  the
       calculation of the buffer length.
       After a successful fflush() or fclose(), the pointer referenced by bufp
       shall contain the address of the buffer, and the variable pointed to by
       sizep  shall  contain  the smaller of the current buffer length and the
       number of bytes for open_memstream(), or the number of wide  characters
       for open_wmemstream(), between the beginning of the buffer and the cur-
       rent file position indicator.
       After a successful fflush() the pointer  referenced  by  bufp  and  the
       variable  referenced  by  sizep  remain valid only until the next write
       operation on the stream or a call to fclose().
RETURN VALUE
       Upon successful completion, these functions shall return a  pointer  to
       the  object  controlling the stream. Otherwise, a null pointer shall be
       returned, and errno shall be set to indicate the error.
ERRORS
       These functions may fail if:
       EINVAL bufp or sizep are NULL.
       EMFILE {FOPEN_MAX} streams are currently open in the calling process.
       ENOMEM Memory for the stream or the buffer could not be allocated.
       The following sections are informative.
EXAMPLES
           #include <stdio.h>
           #include <stdlib.h>
           int
           main (void)
           {
               FILE *stream;
               char *buf;
               size_t len;
               off_t eob;
               stream = open_memstream (&buf, &len);
               if (stream == NULL)
                   /* handle error */ ;
               fprintf (stream, "hello my world");
               fflush (stream);
               printf ("buf=%s, len=%zu\n", buf, len);
               eob = ftello(stream);
               fseeko (stream, 0, SEEK_SET);
               fprintf (stream, "good-bye");
               fseeko (stream, eob, SEEK_SET);
               fclose (stream);
               printf ("buf=%s, len=%zu\n", buf, len);
               free (buf);
               return 0;
           }
       This program produces the following output:
           buf=hello my world, len=14
           buf=good-bye world, len=14
APPLICATION USAGE
       The buffer created by these functions should be freed by  the  applica-
       tion after closing the stream, by means of a call to free().
RATIONALE
       These  functions  are  similar  to fmemopen() except that the memory is
       always allocated dynamically by the function, and the stream is  opened
       only for output.
FUTURE DIRECTIONS
       None.
SEE ALSO
       fclose(), fdopen(), fflush(), fmemopen(), fopen(), free(), freopen()
       The Base Definitions volume of POSIX.1-2008, <stdio.h>, <wchar.h>
COPYRIGHT
       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal  and  Electronics  Engineers,  Inc  and  The  Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum  1  applied.)  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the  referee document. The original Standard can be obtained online
       at http://www.unix.org/online.html .
       Any typographical or formatting errors that appear  in  this  page  are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors,  see  https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                   OPEN_MEMSTREAM(3P)