在C ++中从big-endian二进制文件中检索int的“正确”方法

Question

I have a binary file in big-endian format from which I am retrieving 2-bit and 4-bit integer data. The machine I'm running on is little-endian.

Does anyone have any suggestions or a best-practice on pulling integer data from a known format binary and switching endianness on the fly? I'm not sure that my current solution is even correct:

int myInt;

ifstream dataFile(dataFileLocation, ios::in | ios::binary);
dataFile.seekg(99, ios::beg);  //Pull data starting at byte 100;

//For 4-byte value:
char chunk[4];
dataFile.read(chunk, 4);
myInt = (int)(chunk[0] << 24 | chunk[1] << 16 | chunk[2] << 8 | chunk[3]);

//For 2-byte value:
char chunk[2];
dataFile.read(chunk, 4);
myInt = (int)(chunk[0] << 8 | chunk[1]);

This seems to work fine for 2-byte data but gives what I believe are incorrect values on 4-byte data. I've read about htonl() but from what I've read that's not a smart way to go for flexibility.

Answer 1

Use unsigned integral types only and you'll be fine:

unsigned char buf[4];
infile.read(reinterpret_cast<char*>(buf), 4);

unsigned int b4 = (buf[0] << 24) + ... + (buf[3]);
unsigned int b2 = (buf[0] << 8) + (buf[1]);

Shifting involves type promotions, and indefinite sign extensions (given the implementation-defined nature of char ). Basically you always want everything to be unsigned when manipulating bits.