This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       The buffer created by these functions should be freed by the application after  closing  the  stream,  by
       means of a call to free().

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  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.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                OPEN_MEMSTREAM(3POSIX)

       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  successfully  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 position. In this case a null character for open_memstream() or a
       null wide character for open_wmemstream() shall be appended to the current buffer. 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 current 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().

       After a successful fclose(), the pointer referenced by bufp can be passed to free().

       These functions shall fail if:

       EMFILE {STREAM_MAX} streams are currently open in the calling process.

       These functions may fail if:

       EINVALbufp 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.

       Thefollowingsectionsareinformative.

           #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

       None.

       open_memstream, open_wmemstream — open a dynamic memory buffer stream

       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.

       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.

       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.

fclose(), fdopen(), fflush(), fmemopen(), fopen(), free(), freopen()

       The Base Definitions volume of POSIX.1‐2017, <stdio.h>, <wchar.h>

       #include <stdio.h>

       FILE *open_memstream(char **bufp, size_t *sizep);

       #include <wchar.h>

       FILE *open_wmemstream(wchar_t **bufp, size_t *sizep);