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8.11. Processing Binary Files
8.11.1. Problem
You
want to read 8-bit binary data as 8-bit binary data, i.e., neither as
characters in a particular encoding nor as a text file with any
newline or end-of-file conversions that your I/O library might want
to do.
8.11.2. Solution
Use the binmode function on the
filehandle:binmode(HANDLE);
8.11.3. Discussion
The binmode function lets you specify new I/O
layers for a filehandle. The default layer to specify is
:raw, which removes any layers that would
interfere with binary data. The Solution is thus equivalent to:binmode(HANDLE, ":raw");
except that explicitly specifying :raw only works
on Perl 5.8 and later. The one-argument form of
binmode works on all versions of Perl.
Because Perl
makes :crlf the default if you are on an operating
system that needs it, you should rarely (if ever) need to specify
:crlf in your program. Furthermore, it's generally
not wise to add or remove the :crlf layer once
you've begun reading from the file, as there may be data already read
into buffers that you can't unread. You can, however, safely change
the :encoding(...) layer midstream (when parsing
XML, for example).You should get into the habit of calling binmode
when you open a binary file. This will make your program portable to
systems that might (un)helpfully translate bytes in your binary file
into something unusable.You may specify the I/O layers when you open a
filehandle, rather than using binmode after the
fact:open(FH, "< :raw", $filename); # binary mode
Specify the default set of layers for all subsequently opened input
and output filehandles with the open pragma:use open IN => ":raw"; # binary files
8.11.4. See Also
The PerlIO(3) manpage; the
open and binmode functions in
perlfunc(1) and in Chapter 29 of
Programming Perl; your system's
open(2) and fopen(3)
manpages
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| 8.10. Removing the Last Line of a File |  | 8.12. Using Random-Access I/O |
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