FMEMOPEN(3)                POSIX Programmer's Manual               FMEMOPEN(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
       fmemopen -- open a memory buffer stream

SYNOPSIS
       #include <stdio.h>

       FILE *fmemopen(void *restrict buf, size_t size,
           const char *restrict mode);

DESCRIPTION
       The fmemopen() function shall associate the buffer given by the buf and
       size arguments with a stream. The buf argument shall be either a null
       pointer or point to a buffer that is at least size bytes long.

       The mode argument points to a string. If the string is one of the
       following, the stream shall be opened in the indicated mode. Otherwise,
       the behavior is undefined.

       _r       Open the stream for reading.

       _w       Open the stream for writing.

       _a       Append; open the stream for writing at the first null byte.

       _r+      Open the stream for update (reading and writing).

       _w+      Open the stream for update (reading and writing). Truncate the
               buffer contents.

       _a+      Append; open the stream for update (reading and writing); the
               initial position is at the first null byte.

       Implementations shall accept all mode strings allowed by fopen(), but
       the use of the character 'b' shall produce implementation-defined
       results, where the resulting FILE ** need not behave the same as if 'b'
       were omitted.

       If a null pointer is specified as the buf argument, fmemopen() shall
       allocate size bytes of memory as if by a call to malloc().  This buffer
       shall be automatically freed when the stream is closed.  Because this
       feature is only useful when the stream is opened for updating (because
       there is no way to get a pointer to the buffer) the fmemopen() call may
       fail if the mode argument does not include a ''++''.

       The stream shall maintain a current position in the buffer. This
       position shall be initially set to either the beginning of the buffer
       (for _r and _w modes) or to the first null byte in the buffer (for _a
       modes). If no null byte is found in append mode, the initial position
       shall be set to one byte after the end of the buffer.

       If buf is a null pointer, the initial position shall always be set to
       the beginning of the buffer.

       The stream shall also maintain the size of the current buffer contents;
       use of fseek() or fseeko() on the stream with SEEK_END shall seek
       relative to this size. For modes _r and r+ the size shall be set to the
       value given by the size argument. For modes _w and w+ the initial size
       shall be zero and for modes _a and a+ the initial size shall be:

        *  Zero, if buf is a null pointer

        *  The position of the first null byte in the buffer, if one is found

        *  The value of the size argument, if buf is not a null pointer and no
           null byte is found

       A read operation on the stream shall not advance the current buffer
       position beyond the current buffer size. Reaching the buffer size in a
       read operation shall count as ``end-of-file''. Null bytes in the buffer
       shall have no special meaning for reads. The read operation shall start
       at the current buffer position of the stream.

       A write operation shall start either at the current position of the
       stream (if mode has not specified 'a' as the first character) or at the
       current size of the stream (if mode had 'a' as the first character). If
       the current position at the end of the write is larger than the current
       buffer size, the current buffer size shall be set to the current
       position. A write operation on the stream shall not advance the current
       buffer size beyond the size given in the size argument.

       When a stream open for writing is flushed or closed, a null byte shall
       be written at the current position or at the end of the buffer,
       depending on the size of the contents. If a stream open for update is
       flushed or closed and the last write has advanced the current buffer
       size, a null byte shall be written at the end of the buffer if it fits.

       An attempt to seek a memory buffer stream to a negative position or to
       a position larger than the buffer size given in the size argument shall
       fail.

RETURN VALUE
       Upon successful completion, fmemopen() 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
       The fmemopen() function shall fail if:

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

       The fmemopen() function may fail if:

       EINVAL The value of the mode argument is not valid.

       EINVAL The buf argument is a null pointer and the mode argument does
              not include a ''++'' character.

       EINVAL The size argument specifies a buffer size of zero and the
              implementation does not support this.

       ENOMEM The buf argument is a null pointer and the allocation of a
              buffer of length size has failed.

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

       The following sections are informative.

EXAMPLES
           #include <stdio.h>
           #include <string.h>

           static char buffer[] = "foobar";

           int
           main (void)
           {
               int ch;
               FILE *stream;

               stream = fmemopen(buffer, strlen (buffer), "r");
               if (stream == NULL)
                   /* handle error */;

               while ((ch = fgetc(stream)) != EOF)
                   printf("Got %c\n", ch);

               fclose(stream);
               return (0);
           }

       This program produces the following output:


           Got f
           Got o
           Got o
           Got b
           Got a
           Got r

APPLICATION USAGE
       None.

RATIONALE
       This interface has been introduced to eliminate many of the errors
       encountered in the construction of strings, notably overflowing of
       strings. This interface prevents overflow.

FUTURE DIRECTIONS
       A future version of this standard may mandate specific behavior when
       the mode argument includes 'b'.

       A future version of this standard may require support of zero-length
       buffer streams explicitly.

SEE ALSO
       fdopen(), fopen(), freopen(), fseek(), malloc(), open__memstream()

       The Base Definitions volume of POSIX.1-2017, <stdio.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                          FMEMOPEN(3)
