NAME

       FileHandle::Unget - FileHandle which supports multi-byte unget

SYNOPSIS

         use FileHandle::Unget;

         # open file handle
         my $fh = FileHandle::Unget->new("file")
           or die "cannot open filehandle: $!";

         my $buffer;
         read($fh,$buffer,100);
         print $buffer;

         print <$fh>;

         $fh->close;

DESCRIPTION

       FileHandle::Unget operates exactly the same as FileHandle, except that it provides a
       version of ungetc that allows you to unget more than one character.  It also provides
       ungets to unget a string.

       This module is useful if the filehandle refers to a stream for which you can't just
       "seek()" backwards. Some operating systems support multi-byte "ungetc()", but this is not
       guaranteed. Use this module if you want a portable solution. In addition, on some
       operating systems, eof() will not be reset if you ungetc after having read to the end of
       the file.

       NOTE: Using "sysread()" with "ungetc()" and other buffering functions is still a bad idea.

METHODS

       The methods for this package are the same as those of the FileHandle package, with the
       following exceptions.

       new ( ARGS )
           The constructor is exactly the same as that of FileHandle, except that you can also
           call it with an existing IO::Handle object to "attach" unget semantics to a pre-
           existing handle.

       $fh->ungetc ( ORD )
           Pushes a character with the given ordinal value back onto the given handle's input
           stream. This method can be called more than once in a row to put multiple values back
           on the stream. Memory usage is equal to the total number of bytes pushed back.

       $fh->ungets ( BUF )
           Pushes a buffer back onto the given handle's input stream. This method can be called
           more than once in a row to put multiple buffers of characters back on the stream.
           Memory usage is equal to the total number of bytes pushed back.

           The buffer is not processed in any way--managing end-of-line characters and whatnot is
           your responsibility.

       $fh->buffer ( [BUF] )
           Get or set the pushback buffer directly.

       $fh->input_record_separator ( STRING )
           Get or set the per-filehandle input record separator. If an argument is specified, the
           input record separator for the filehandle is made independent of the global $/. Until
           this method is called (and after clear_input_record_separator is called) the global $/
           is used.

           Note that a return value of "undef" is ambiguous. It can either mean that this method
           has never been called with an argument, or it can mean that it was called with an
           argument of "undef".

       $fh->clear_input_record_separator ()
           Clear the per-filehandle input record separator. This removes the per-filehandle input
           record separator semantics, reverting the filehandle to the normal global $/
           semantics.

       tell ( $fh )
           "tell" returns the actual file position minus the length of the unget buffer.  If you
           read three bytes, then unget three bytes, "tell" will report a file position of 0.

           Everything works as expected if you are careful to unget the exact same bytes which
           you read.  However, things get tricky if you unget different bytes.  First, the next
           bytes you read won't be the actual bytes on the filehandle at the position indicated
           by "tell".  Second, "tell" will return a negative number if you unget more bytes than
           you read. (This can be problematic since this function returns -1 on error.)

       seek ( $fh, [POSITION], [WHENCE] )
           "seek" defaults to the standard seek if possible, clearing the unget buffer if it
           succeeds. If the standard seek fails, then "seek" will attempt to seek within the
           unget buffer. Note that in this case, you will not be able to seek
           backward--FileHandle::Unget will only save a buffer for the next bytes to be read.

           For example, let's say you read 10 bytes from a pipe, then unget the 10 bytes.  If you
           seek 5 bytes forward, you won't be able to read the first five bytes.  (Otherwise this
           module would have to keep around a lot of probably useless data!)

COMPATIBILITY

       To test that this module is indeed a drop-in replacement for FileHandle, the following
       modules were modified to use FileHandle::Unget, and tested using "make test". They have
       all passed.

BUGS

       There is a bug in Perl on Windows that is exposed if you open a stream, then check for
       eof, then call binmode. For example:

         # First line
         # Second line

         open FH, "$^X -e \"open F, '$0';binmode STDOUT;print <F>\" |";

         eof(FH);
         binmode(FH);

         print "First line:", scalar <FH>, "\n";
         print "Second line:", scalar <FH>, "\n";

         close FH;

       One solution is to make sure that you only call binmode immediately after opening the
       filehandle. I'm not aware of any workaround for this bug that FileHandle::Unget could
       implement. However, the module does detect this situation and prints a warning.

       Contact david@coppit.org for bug reports and suggestions.

AUTHOR

       David Coppit <david@coppit.org>.

LICENSE

       This code is distributed under the GNU General Public License (GPL) Version 2.  See the
       file LICENSE in the distribution for details.

SEE ALSO

       Mail::Mbox::MessageParser for an example of how to use this package.