Convert four bytes to Integer using C++

Question

I am trying to convert 4 bytes to an integer using C++.

This is my code:

int buffToInteger(char * buffer)
{
    int a = (int)(buffer[0] << 24 | buffer[1] << 16 | buffer[2] << 8 | buffer[3]);
    return a;
}

The code above works in almost all cases, for example: When my buffer is: "[\x00, \x00, \x40, \x00]" the code will return 16384 as expected.

But when the buffer is filled with: "[\x00, \x00, \x3e, \xe3]", the code won't work as expected and will return "ffffffe1".

Does anyone know why this happens?

Answer 1

Your buffer contains signed characters. So, actually, buffer[0] == -29, which upon conversion to int gets sign-extended to 0xffffffe3, and in turn (0x3e << 8) | 0xffffffe3 == 0xffffffe3.

You need ensure your individual buffer bytes are interpreted unsigned, either by declaring buffer as unsigned char *, or by explicitly casting:

int a = int((unsigned char)(buffer[0]) << 24 |
            (unsigned char)(buffer[1]) << 16 |
            (unsigned char)(buffer[2]) << 8 |
            (unsigned char)(buffer[3]));

Answer 2

In the expression buffer[0] << 24 the value 24 is an int, so buffer[0] will also be converted to an int before the shift is performed.

On your system a char is apparently signed, and will then be sign extended when converted to int.