On Fri, Aug 26, 2005 at 12:52:17AM -0400, Ian C. Blenke wrote:
> Paul M Foster wrote:
>
> >Anyone know how to pull data out of a file and interpret it as binary in
> >PHP? Specifically, I have four bytes in a file whose contents I want to
> >bit-shift left by varying amounts, and then store the results as a
> >number. The raw data is binary. In python, it looks like this:
> >
> >s0 = ord(four_byte_string[0])
> >s1 = ord(four_byte_string[1])
> >s2 = ord(four_byte_string[2])
> >s3 = ord(four_byte_string[3])
> >return (s3 << 24) | (s2 << 16) | (s1 << 8) | s0
> >
> >and the value returned should be a number. Anyone know how to accomplish
> >a similar thing in PHP?
> >
> >
> Depending on the binary endianness, you could always just fread() in the
> 4 bytes into a string and settype() it to an integer (or do a "+= 0" to
> convert it).
>
> If you read in each byte, a character at a time, you can always use
> pack() to ensure the endianness.
>
> This is the kind of thing pack() and unpack() are for (vaguely similar
> to the perl equivalent functions):
>
> $handle = fopen("/some/path/binary_file.bin", "rb");
> $s0 = fread($handle, 1);
> $s1 = fread($handle, 1);
> $s2 = fread($handle, 1);
> $s3 = fread($handle, 1);
> $binary_string = pack ('C4', $s0, $s1, $s2, $s3);
>
> // This is just an example of how to use unpack()
> $value=unpack('Linteger',$binary_string);
> $integer=$value['integer'];
> $integer+=0; // $integer is now a number containing the unpacked
> Long (32bit) value parsed from $binary_string
>
> // Alternatively, $binary_string is already a 32bit integer, just
> make sure php treats it as a number. This _should_ work.
> $binary_string+=0;
>
> fclose($handle);
>
> I think this is correct, but I've not written php in a while, and the
> the above was pasted together off of some google searches without
> testing any of it.
I actually found this before you emailed the list, but congratulations
for being the guy who could answer the question.
The solution actually looks like this:
$header = fread($fd, 12);
$hd = unpack('Vjunk/Vnrecs/vhlen/vrlen', $header);
extract($hd);
which yields $junk, $nrecs, $hlen and $rlen as numeric variables. The
'V' (or 'v') versus the 'N' (or 'n') in unpack allows you to simply
control endianness. It also yields $hd[junk], $hd[nrecs], etc. There's
no need to use pack() first, since the assumption is that the data is
already packed.
Unfortunately, the documentation for unpack() seems to indicate you can
use unpack('V/V/v/v', $header), but it apparently doesn't work without
actually giving a variable name after each conversion spec.
Anyway, thanks for the assist.
Paul
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