a) You only need to worry about "endianness" (i.e., byte-swapping) if the data was created on a big-endian machine and is being processed on a little-endian machine, or vice versa. There are many ways this can occur, but here are a couple of examples.
- You receive data on a Windows machine via a socket. Windows employs a little-endian architecture while network data is "supposed" to be in big-endian format.
- You process a data file that was created on a system with a different "endianness."
In either of these cases, you'll need to byte-swap all numbers that are bigger than 1 byte, e.g., shorts, ints, longs, doubles, etc. However, if you are always dealing with data from the same platform, endian issues are of no concern.
b) Based on your question, it sounds like you have a char pointer and want to extract the first 4 bytes as an int and then deal with any endian issues. To do the extraction, use this:
int n = *(reinterpret_cast<int *>(myArray)); // where myArray is your data
Obviously, this assumes myArray is not a null pointer; otherwise, this will crash since it dereferences the pointer, so employ a good defensive programming scheme.
To swap the bytes on Windows, you can use the ntohs()/ntohl() and/or htons()/htonl() functions defined in winsock2.h. Or you can write some simple routines to do this in C++, for example:
inline unsigned short swap_16bit(unsigned short us)
{
return (unsigned short)(((us & 0xFF00) >> 8) |
((us & 0x00FF) << 8));
}
inline unsigned long swap_32bit(unsigned long ul)
{
return (unsigned long)(((ul & 0xFF000000) >> 24) |
((ul & 0x00FF0000) >> 8) |
((ul & 0x0000FF00) << 8) |
((ul & 0x000000FF) << 24));
}