Question

i dunno where is the problem exactly, but it seems when i pass ‘Memory’ by reference, it doesn’t work. im trying to Read a Word From Memory in Big Endian and Increment Data by 2

here is how i do it

WORD ReadBigEndianWORD(char **Data)
{
    WORD Result = (unsigned char) *Data[0];
    Result <<= 8;
    Result |= (unsigned char) *Data[1];
    *Data++
    return Result;
}

and i call it like this

char *Memory = ........;
WORD MyWord = ReadBigEndianWORD(&Memory);

the code above doesn’t work (works if Function Argment is ‘char *Data’ and Data passed as ‘Memory’);

and is this the best way to read Big Endian from Memory?

thanks;

Answer 1

Your code is failing because of the operation precedence of the * operator and the [] operation. The [] has higher precedence, so you need to change your code to:

WORD ReadBigEndianWORD(char **Data)
{
    WORD Result = (unsigned char) (*Data)[0];
    Result <<= 8;
    Result |= (unsigned char) (*Data)[1];
    (*Data) += sizeof(WORD);
    return Result;
}

The line with *Data[0] ends up working correctly because you’re adding an offset of zero. The second *Data[1] will either crash our give you bad data because it’s reading for uninitialized memory. *Data[1] is equivalent to \*(\*(Data + sizeof(char**))), so it’s tacking the the location where Memory is stored and moving past that.

The best way to see what’s happening is to use your original ReadBigEndianWORD and change the calling code to:

char *Memory[2] = { "........", ",,,,,,,,," };
short Myshort = ReadBigEndianshort(Memory);

If you do that you’ll see that *Data[1] is pointing to Memory[1].

And then to move your Data pointer to the next work you need to increment it by the sizeof a WORD, not the size of char** which is what ++ will do.

But as everyone else has said you should change your method to take a char* and then worry about moving around in your buffer in the calling code. That will make your function cleaner and clearer.