FileHandle::Unget(3pm)

1FileHandle::Unget(3)  User Contributed Perl Documentation FileHandle::Unget(3)
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NAME

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

9         use FileHandle::Unget;
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11         # open file handle
12         my $fh = FileHandle::Unget->new("file")
13           or die "cannot open filehandle: $!";
14
15         my $buffer;
16         read($fh,$buffer,100);
17         print $buffer;
18
19         print <$fh>;
20
21         $fh->close;
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DESCRIPTION

24       FileHandle::Unget operates exactly the same as FileHandle, except that
25       it provides a version of ungetc that allows you to unget more than one
26       character.  It also provides ungets to unget a string.
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28       This module is useful if the filehandle refers to a stream for which
29       you can't just seek() backwards. Some operating systems support multi-
30       byte ungetc(), but this is not guaranteed. Use this module if you want
31       a portable solution. In addition, on some operating systems, eof() will
32       not be reset if you ungetc after having read to the end of the file.
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34       NOTE: Using sysread() with ungetc() and other buffering functions is
35       still a bad idea.
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METHODS

38       The methods for this package are the same as those of the FileHandle
39       package, with the following exceptions.
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41       new ( ARGS )
42           The constructor is exactly the same as that of FileHandle, except
43           that you can also call it with an existing IO::Handle object to
44           "attach" unget semantics to a pre-existing handle.
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46       $fh->ungetc ( ORD )
47           Pushes a character with the given ordinal value back onto the given
48           handle's input stream. This method can be called more than once in
49           a row to put multiple values back on the stream. Memory usage is
50           equal to the total number of bytes pushed back.
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52       $fh->ungets ( BUF )
53           Pushes a buffer back onto the given handle's input stream. This
54           method can be called more than once in a row to put multiple
55           buffers of characters back on the stream.  Memory usage is equal to
56           the total number of bytes pushed back.
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58           The buffer is not processed in any way--managing end-of-line
59           characters and whatnot is your responsibility.
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61       $fh->buffer ( [BUF] )
62           Get or set the pushback buffer directly.
63
64       $fh->input_record_separator ( STRING )
65           Get or set the per-filehandle input record separator. If an
66           argument is specified, the input record separator for the
67           filehandle is made independent of the global $/. Until this method
68           is called (and after clear_input_record_separator is called) the
69           global $/ is used.
70
71           Note that a return value of "undef" is ambiguous. It can either
72           mean that this method has never been called with an argument, or it
73           can mean that it was called with an argument of "undef".
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75       $fh->clear_input_record_separator ()
76           Clear the per-filehandle input record separator. This removes the
77           per-filehandle input record separator semantics, reverting the
78           filehandle to the normal global $/ semantics.
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80       tell ( $fh )
81           "tell" returns the actual file position minus the length of the
82           unget buffer.  If you read three bytes, then unget three bytes,
83           "tell" will report a file position of 0.
84
85           Everything works as expected if you are careful to unget the exact
86           same bytes which you read.  However, things get tricky if you unget
87           different bytes.  First, the next bytes you read won't be the
88           actual bytes on the filehandle at the position indicated by "tell".
89           Second, "tell" will return a negative number if you unget more
90           bytes than you read. (This can be problematic since this function
91           returns -1 on error.)
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93       seek ( $fh, [POSITION], [WHENCE] )
94           "seek" defaults to the standard seek if possible, clearing the
95           unget buffer if it succeeds. If the standard seek fails, then
96           "seek" will attempt to seek within the unget buffer. Note that in
97           this case, you will not be able to seek backward--FileHandle::Unget
98           will only save a buffer for the next bytes to be read.
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100           For example, let's say you read 10 bytes from a pipe, then unget
101           the 10 bytes.  If you seek 5 bytes forward, you won't be able to
102           read the first five bytes.  (Otherwise this module would have to
103           keep around a lot of probably useless data!)
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COMPATIBILITY

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

111       There is a bug in Perl on Windows that is exposed if you open a stream,
112       then check for eof, then call binmode. For example:
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114         # First line
115         # Second line
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117         open FH, "$^X -e \"open F, '$0';binmode STDOUT;print <F>\" |";
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119         eof(FH);
120         binmode(FH);
121
122         print "First line:", scalar <FH>, "\n";
123         print "Second line:", scalar <FH>, "\n";
124
125         close FH;
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127       One solution is to make sure that you only call binmode immediately
128       after opening the filehandle. I'm not aware of any workaround for this
129       bug that FileHandle::Unget could implement. However, the module does
130       detect this situation and prints a warning.
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132       Contact david@coppit.org for bug reports and suggestions.
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AUTHOR

135       David Coppit <david@coppit.org>.
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LICENSE

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