02-25-2008
Parsing Binary
I have a binary file a particular format.
It contains the Length Bytes and the Type bytes i.e the first four bytes if the file indicate the length of the Type which is to follow.
for eg, if the int value of the first four bytes is 80, then it means that the length of the following "Type" is 80. after i read the 80 bytes, the next four bytes again refer to the length of the next "Type"
I need to open the binary file, read the first four bytes, and then get the obtain the the bytes equaling the length obtained and keep doing this till the end of the file.
The problem i am having is that sometime the length of the file that it detects it wrong and that throws off the entire reading of the file
This is how i am reading the 4 bytes for length and then getting the long value
fread (Len, 1, 4, fp_bin);
long Length =(((((Len[0] << 8) + Len[1]) << 8) + Len[2]) << 8) + Len[3];
where Len is the char array containing the 4 bytes
this is how i am reading in the next 'Length' number of bytes
read(Data, 1, Length, fp_bin);
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bytes(3perl) Perl Programmers Reference Guide bytes(3perl)
NAME
bytes - Perl pragma to force byte semantics rather than character semantics
NOTICE
This pragma reflects early attempts to incorporate Unicode into perl and has since been superseded. It breaks encapsulation (i.e. it
exposes the innards of how the perl executable currently happens to store a string), and use of this module for anything other than
debugging purposes is strongly discouraged. If you feel that the functions here within might be useful for your application, this possibly
indicates a mismatch between your mental model of Perl Unicode and the current reality. In that case, you may wish to read some of the perl
Unicode documentation: perluniintro, perlunitut, perlunifaq and perlunicode.
SYNOPSIS
use bytes;
... chr(...); # or bytes::chr
... index(...); # or bytes::index
... length(...); # or bytes::length
... ord(...); # or bytes::ord
... rindex(...); # or bytes::rindex
... substr(...); # or bytes::substr
no bytes;
DESCRIPTION
The "use bytes" pragma disables character semantics for the rest of the lexical scope in which it appears. "no bytes" can be used to
reverse the effect of "use bytes" within the current lexical scope.
Perl normally assumes character semantics in the presence of character data (i.e. data that has come from a source that has been marked as
being of a particular character encoding). When "use bytes" is in effect, the encoding is temporarily ignored, and each string is treated
as a series of bytes.
As an example, when Perl sees "$x = chr(400)", it encodes the character in UTF-8 and stores it in $x. Then it is marked as character data,
so, for instance, "length $x" returns 1. However, in the scope of the "bytes" pragma, $x is treated as a series of bytes - the bytes that
make up the UTF8 encoding - and "length $x" returns 2:
$x = chr(400);
print "Length is ", length $x, "
"; # "Length is 1"
printf "Contents are %vd
", $x; # "Contents are 400"
{
use bytes; # or "require bytes; bytes::length()"
print "Length is ", length $x, "
"; # "Length is 2"
printf "Contents are %vd
", $x; # "Contents are 198.144"
}
chr(), ord(), substr(), index() and rindex() behave similarly.
For more on the implications and differences between character semantics and byte semantics, see perluniintro and perlunicode.
LIMITATIONS
bytes::substr() does not work as an lvalue().
SEE ALSO
perluniintro, perlunicode, utf8
perl v5.14.2 2010-12-30 bytes(3perl)