bionic (3) FileHandle::Unget.3pm.gz

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.