Why does the system function always return a shifted exit status in C++? [duplicate]

Question

I want to launch a script and get the exit status in C++. But I noticed that the exit status of a system command always has a left shift of 8 bits. Why does the system function do that?

Sample code:

#include <iostream>

using namespace std;

int main()
{
    int exit_code_0 = system("exit 0") >> 8;
    int exit_code_1 = system("exit 1") >> 8;
    int exit_code_2 = system("exit 2") >> 8;
    int exit_code_3 = system("exit 3") >> 8;
    int exit_code_4 = system("exit 4") >> 8;
    int exit_code_255 = system("exit 255")>> 8;
    cout<<"Exit code was: " << exit_code_0 <<", Expected: 0"<< endl;
    cout<<"Exit code was: " << exit_code_1 <<", Expected: 1"<< endl;
    cout<<"Exit code was: " << exit_code_2 <<", Expected: 2"<< endl;
    cout<<"Exit code was: " << exit_code_3 <<", Expected: 3"<< endl;
    cout<<"Exit code was: " << exit_code_4 <<", Expected: 4"<< endl;
    cout<<"Exit code was: " << exit_code_255 <<", Expected: 255"<< endl;
    
    return 0;
}

Test results:

Exit code was: 0, Expected: 0                                                                                                                                                       
Exit code was: 1, Expected: 1                                                                                                                                                       
Exit code was: 2, Expected: 2                                                                                                                                                       
Exit code was: 3, Expected: 3                                                                                                                                                       
Exit code was: 4, Expected: 4                                                                                                                                                       
Exit code was: 255, Expected: 255

Answer 1

In the C and C++ standards, system returns an implementation-defined value.

In POSIX systems, the return value contains a combination of different values, which can be extracted with macros. To get the program's exit status, you use WEXITSTATUS(return_value), which on your system is defined to be a right shift by 8. The lower 8 bits contain other values (letting you determine, for example, if the program exited normally or due to a signal, etc.)

Answer 2

You are supposed to read the return value using

WEXITSTATUS(code)

https://man7.org/linux/man-pages/man3/system.3.html