OPEN_MEMSTREAM(3)          POSIX Programmer's Manual         OPEN_MEMSTREAM(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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
       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().

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 shall fail if:

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

       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
       application 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.1-2017, <stdio.h>, <wchar.h>

COPYRIGHT
       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(3)
