open_memstream(3p) — Linux manual page


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

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

       The stream associated with a call to open_memstream() shall be

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

       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         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

ERRORS         top

       These functions shall fail if:

       EMFILE {STREAM_MAX} streams are currently open in the calling

       These functions may fail if:

       EINVAL bufp or sizep are NULL.

       EMFILE {FOPEN_MAX} streams are currently open in the calling

       ENOMEM Memory for the stream or the buffer could not be

       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

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.1‐2017, stdio.h(0p),

COPYRIGHT         top

       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 .

       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               2017                OPEN_MEMSTREAM(3P)

Pages that refer to this page: stdio.h(0p)wchar.h(0p)fclose(3p)fdopen(3p)fflush(3p)fmemopen(3p)fopen(3p)freopen(3p)