Reading binary data

[Poda]

Hi, guys.  I'm trying to wrap my head around trying to read a header in a binary file in php.  So, far, I have been able to get the data into a string using file_get_contents.  Now, I think I need to use unpack, but it's a bit confusing to me still.  For example, my file starts by having a 1 byte version number, 4 bytes to represent a number of walls, then "number of walls" * 40 bytes for the actual walls.  But, the problem is, the wall itself is a structure composed of various elements to make up that 40 bytes, from which I'll need to pick out a specific area of interest.  I can do this easily in C, but in php. I can't think how to grab the info.  Any ideas?  Maybe just how to read 1 byte, 4 bytes, then "number of walls" * 40 bytes will get me in the right direction.

[RussellReal]

show us an example file?

[Poda]

Hi.  My example isn't 100% accurate to the file.  But, basically, they are .map files.  Here is how they start:

4 bytes-version

4 bytes-position x

4 bytes-position y

4 bytes-position z

2 bytes-angle

2 bytes-current sector

2 bytes-number of sectors

40 bytes * number of sectors-the sectors themselves.

 

I attached an example map file.  Really, it's just the 40 bytes * number of sectors that I can't seem to wrap my head around.  I can see how to read the other stuff with the unpack function.

 

[attachment deleted by admin]

[RussellReal]

lol I'm trying to unpack it, but I'm having no luck maybe someone here whose done more work with unpack can get it to work

 

you can probably use substr to manually get the bytes you need

[Alex]

Are the bytes signed or unsigned?

 

Edit: Assuming that they're signed, something like this should work:

 

$file = file_get_contents('somefile.map');
$version = current(unpack('i', substr($file, 0, 4)));
$position_x = current(unpack('i', substr($file, 4, 4)));
// ...
$number_of_sectors = current(unpack('s', substr($file, 22, 2)));

for($i = 0;$i < $number_of_sectors;++$i) {
    // read 40 bytes
    // $bytes = substr($file, $i * 40 + 24, 40);
}

[Poda]

Ok, here is what I got so far.  I tested with my C++ program and it checks out ok

 

except the number of walls is wrong.  Here is my code

 

<?php

#read the file

$file = "1hk.map";
$fp = fopen ($file, 'rb') or die ("File <i>$file</i> cannot be opened.");

$data = fread ($fp, 500000) or die ("Could not read data from file <i>$file</i>");

#everything up to the number of sectors.

$header_format = 'Lversion/Lposx/Lposy/Lposz/Sang/Scurrentsector/Snumsectors';
$header = unpack ($header_format, $data);

#now, read the number of walls after the sectors by skipping

$sectors = $header['numsectors'] * 40;

$header_format = 'Lversion/Lposx/Lposy/Lposz/Sang/Scurrentsector/Snumsectors/x$sectors/Snumwalls';

# Unpack the header data
$header = unpack ($header_format, $data);

# Display the data stored in $data
print_r ($header);



?>

 

Basically, it doesn't seem to like my skipping of the number of sector bytes.  But, if I give it the literal number of $sectors, it works ok.

[Poda]

In other words, in my part where I put

 

$header_format = 'Lversion/Lposx/Lposy/Lposz/Sang/Scurrentsector/Snumsectors/x$sectors/Snumwalls';

 

If I change $sectors to what its literal value is at that point of 14440, it works ok.  I don't get it.  Can you not use variables inside this expression?

[Poda]

Your substr thing works, dude.  Thanks.