fmemopen(3p) — Linux manual page

PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

       fmemopen — open a memory buffer stream

SYNOPSIS         top

       #include <stdio.h>

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

DESCRIPTION         top

       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         top

       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         top

       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         top

           #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         top

       None.

RATIONALE         top

       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         top

       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         top

       fdopen(3p), fopen(3p), freopen(3p), fseek(3p), malloc(3p),
       open_memstream(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 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(3P)

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