判断系统的字节顺序

Why Worry About Byte Order

In general, the underlying byte order of the processor is completely transparent to the programmer. However, there can be a problem, for example, when data is exchanged with another system, since the other system may interpret multi-byte values differently.

For example, since it is not possible to predict the type of system at either end of the network, network protocols must define the byte order that is used for multi-byte values in their headers. This is called the network byte order, and for TCP/IP, it is big endian. Thus, the sending system converts the data from it local byte order to the network byte order. Then, the receiving system converts the data from network byte order to its local byte order. In practice, if either system uses the same byte order as the network byte order, the conversion operation is optimized out and no conversion takes place.

Another example is the USB protocol, which defines that multi-byte values will use the little endian byte order.

union{
	int i;
	char c[sizeof(int)];
} endian;

endian.i=1;
if(endian.c[0]==1){
	std::cout<<"little"<<std::endl;
}else{
	std::cout<<"big"<<std::endl;
}