Reading internal file offsets in binary files

Ant

Hi,

I'm relatively new to PHP, but have reasonable experience with other languages. But I'm sortoff stuck at this moment.

The case is as follows:

I have a file (Exif 2.2 to be precise) and I want to read some specific metadata from it. The location of the metadata is pointed to by several offsets in the header of the file.

The problem is that I can get the offsets, but I can't read them as integers which I need to be able to 'seek()' in the file. I tried type casting etc, but that didn't work. I still end up with some sort of string (text) value.

The only solution I could think of is to create some sort of lookup table to be used to recreate the offsets byte for byte. But that's pretty ugly for something this trivial.

Am I missing something?

Thanks in advance for any pointers (no pun intended)

Ant

ednark

no your not missing anything... php tries hard to make type casting not necessary... it will usually figure out when it needs to use a string as a number, etc...

however with-out a code example... its hard to say what going on...

i can do this fine... so your code must be different than mine...

<?php
header('Content-type: text/plain');
/// absolute offset of X : data
$data = '5:000X000';
echo "Data to be read into file (first char is offset of where X should be): \n". $data ."\n\n";

$fp = fopen( './fseek.data', 'w+' );
fwrite( $fp, $data );
fclose( $fp );

$fp  = fopen( './fseek.data', 'r' );
$offset = fread( $fp, 1 );

echo "Offset as read by fread() = (". gettype($offset) .")  ". $offset ."\n";
fseek($fp,$offset,SEEK_SET);
$x = fread( $fp, 1 );
echo "Char as offset ($offset) = (". gettype($x) .") ". $x ."\n\n";


settype($offset,'integer');
echo "Offset after type conversion = (". gettype($offset) .") ". $offset ."\n";
fseek($fp,$offset,SEEK_SET);
$x = fread( $fp, 1 );
echo "Char as offset ($offset) = (". gettype($x) .") ". $x ."\n";

fclose( $fp );

?>

Ant

Thanks for your reply Ednark,

However this is not exactly what I mean/need. If you read '5' from $data, it is interpreted as value 5 when you typecast it to INT. But actually what I need it to represent is the ASCII or binary value, in this case 53 (dec) for the caracter 5 -not- the face value.

I need to get at them bits 🙂

So if I read 'ÿØ' from the file, how get do I get PHP to give me 0xFFD8 or 65496 as being the value it represents? The only 'solution' I can think of is to create some sort of lookup-table and to piece my values together byte for byte.

I'm just starting to suspect that PHP is not meant to be able to do this.

Ant

ednark

ok this was an answer to a previous post.. but ive forgotten where i posted this... so ill give it to you here instead of referring you to the other post...

you may want to read up on the [man]unpack[/man] function

here is an example of it in action: yours may be different slightly.. and this does not give you the string representation of that the hex looks like.. so maybe this is not relevant either


header('Content-type: text/plain');

$data = pack( 'd', 2.22E2 );

$fp = fopen( './binread.data', 'w+' );
fwrite( $fp, $data );
fclose( $fp );


$fp  = fopen( './binread.data', 'r' );
$val = fread( $fp, 8 );
fclose( $fp );

$result     = unpack( 'd', $val );
$unpacked   = $result[NULL];

echo gettype($unpacked) ." : ". $unpacked ."\n";

Ant

Ah, there we go. I read the link and this is what I was looking for. Hmm, maybe I should have learned Perl first since the functions pack() and unpack() where derived from there... But nooo I had to go the PHP way 😃

Thank you very much Ednark. You've helped me on my way!

Take care

Ant