bionic (3) open_memstream.3posix.gz

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  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

       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 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.1‐2008, <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 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
       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.kernel.org/doc/man-pages/reporting_bugs.html .