How do you extract bytes from a file into simple data types?

Question

I've tried everything I can think of and can't get anything to work. I have a binary file I've written in VB.Net which basically consists of an integer, (in binary of course) that tells me the array size for the following data, then the floats as binary data. The file writes just fine from VB.Net, and I can read it back in through Visual C++ just fine using the following code:

ifstream output("c:\\out.ipv", ios::in | ios::binary);
UInt32 len;
UInt32 *ptr2 = (UInt32*)&len;
output.read((char*)ptr2, 4);

This returns the correct value of 456780, bytes are: 76, 248, 6, 0. When I run the exact same code on my iPad, I get 1043089572. If I use the alternate method below:

NSData *data = [[NSData alloc] initWithContentsOfFile:filePath];
UInt32 num;
const NSRange numV = {0, 4};
[data getBytes:&num range:numV];

This code returns a different value, 124724, and I'm not sure how to read what the exact bytes are that are getting pulled from the file. That's something else I was trying to figure out but couldn't get working. Any idea why the same method that works in Visual C++ won't work on the iPad? I'm really at a loss on this one.

Answer 1

OK, something really strange is going on with my data. I just looked at the raw byte values in both Visual c++ and objective-c, and they don't agree at all. I'm only reading the first four bytes of the file and looking at their values. I'm assuming at this point that I'm not reading them in correctly, but I don't know what I'm missing here. The Visual C++ code I'm using to look at the byte values is below:

ifstream input("c:\\out.ipv", ios::in | ios::binary);
Byte tmp[4];
input.read((char*)&tmp[0], 4);

The values in the tmp array are:

76
248
6
0

If I do the same thing in objective-c:

ifstream input([filePath UTF8String], ios::in | ios::binary);
Byte tmp[4];
input.read((char*)&tmp[0], 4);

I get:

164
72
44
62

What gives? I would have at least expected to get the same byte values. The file containing the four bytes I am having trouble with is here: newout1.ipv

EDIT:

I realized where the 164,72,44,62 byte values are coming from: those are the intial values the Byte array has before I put anything in it. For some reason the line:

input.read((char*)&tmp[0], 4);

isn't doing anything. Any ideas why it's not reading from the file like it should?

FINAL EDIT:

OK, I probably shouldn't post the answer to this since it makes me look really dumb, but I don't want anyone reading these posts to get confused. So the arrays and objects were always returning the same values no matter what, which also happened to be whatever values they had when they were allocated. I had one too many .'s in my filename, so it was trying to read in out..ipv rather than out.ipv. Once I fixed the filename, everything worked exactly how I expected it to. Sorry for the confusion, and thanks for everyones help.

Answer 2

This sounds like an endian issue. You can use any of the functions in <libkern/OSByteOrder.h> to read data in a specified endianness. In your case, you may want to do something like

NSInputStream *istream = [NSInputStream inputStreamWithFileAtPath:filePath];
UInt32 num = 0;
if (istream) {
    uint8_t buffer[4];
    if ([istream read:buffer maxLength:4] == 4) {
        num = OSReadLittleInt32(buffer, 0);
    } else {
        // there weren't 4 bytes in the file
    }
} else {
    // the file could not be opened
}