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

PROLOG         top

       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         top

       open_memstream, open_wmemstream — open a dynamic memory buffer stream

SYNOPSIS         top

       #include <stdio.h>

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

       #include <wchar.h>

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

DESCRIPTION         top

       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-

       The stream associated with a call to open_wmemstream() shall be wide-

       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().

RETURN VALUE         top

       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         top

       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         top

           #include <stdio.h>
           #include <stdlib.h>

           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


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

RATIONALE         top

       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.



SEE ALSO         top

       fclose(3p), fdopen(3p), fflush(3p), fmemopen(3p), fopen(3p),
       free(3p), freopen(3p)

       The Base Definitions volume of POSIX.1‐2008, stdio.h(0p), wchar.h(0p)

COPYRIGHT         top

       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 Electrical 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 .

       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 .

IEEE/The Open Group                 2013                  OPEN_MEMSTREAM(3P)