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Question

I'm trying to access some data member inside a binary file, using a struct definition.

Inside thi file, it should contain a struct member called fxID. In the following code, I'm trying to access fxID, but it's not working, it's returning an incorrect number. Even more strangely, if I change fxID to fxVersion, I get the same number, so it does appear that the printf is not picking up the correct data at all.

FILE *p;
struct myStruct x;
p=fopen("myfile.fxb","rb");

size_t n = fread(&x, sizeof(x), 1, p);
if (n != 1) {
    // Some error message
    //printf("%i", sizeof(x));
} else {
    printf("\n\nID:%d\n",  x.fxID);
}

Any idea why I can't access the data inside fxID? I'm getting strange results depending on what type I use in the printf statement. For instance, I get the number 1606416148 if I use printf("%d", x.fxID); However, this is wrong, because I believe the fxID is 1447514692. Is this because of the endianness? I know the file is big endian and it mentions something about needing to convert it in the header file I linked to.

Could it be due to using Int? or the wrong type perhaps?

Answer 1

Endianess must be corrected for sure if the one used in the file is different from the one used by the platform. If the format is stored using big endian layout then, if you are working on x64, you will have to swap the bytes of each multi-byte field.

But this is not enough to guarantee that the layout of your struct is the same of the one used in the file format as it could contain padding and alignment requirements, so in general if you want to deserialize binary data directly inside a struct you must take care of multiple things.

In any case if the obtained result is 16064 1 6148 and you were expecting 1447514692 then endianness is not the only problem since both in hex are respectively 0x5FBFF714 and 0x56475244 which are not the same value with different endianness.

You are using &x.fxID which returns the address of the fxID field inside the struct, which is not the value, try to use x.fxID directy.