C++: Why does std::system("exit 1") return 256?

Question

I want to run a little UNIX shell script inside my C++ program and I want to capture the exit code of the shell script. But the value returned by std::system is not what I expect it to be:

#include <iostream>
#include <cstdlib>

int main()
{
  std::cout << std::system("echo Hello >/dev/null") << std::endl;
  std::cout << std::system("which does_not_exisit 2>/dev/null") << std::endl;

  std::cout << std::system("exit 0") << std::endl;
  std::cout << std::system("exit 1") << std::endl;
  std::cout << std::system("exit 2") << std::endl;
  std::cout << std::system("exit 3") << std::endl;
  std::cout << std::system("echo exit 4 | bash") << std::endl;

  return 0;
}

On my Linux box this produces:

It seems like all exit codes greater than 0 are multiplied by 256. What's the reason for this behavior? Is this portable across UNIX like operating systems?

Look at http://stackoverflow.com/questions/10931134/return-value-of-system-function-call-in-c-used-to-run-a-python-program — pcarter, Oct 23 '15 at 19:46
From http://en.cppreference.com/w/cpp/utility/program/system : the returned value is implementation-defined. — vsoftco, Oct 23 '15 at 19:53
[This](http://stackoverflow.com/a/808995/1062948) ought to answer the mysterious multiplication by 256. — crayzeewulf, Oct 23 '15 at 20:16

score 6 · Answer 1 · edited May 23 '17 at 12:29

6

And the answer is:

  int status = std::system("exit 3");
  std::cout << WEXITSTATUS(status) << std::endl;

Which returns:

For more information visit:

Return value of system() function call in C++, used to run a Python program

edited May 23 '17 at 12:29

Community

1
1

answered Oct 23 '15 at 20:03

Linoliumz

2,381
3
28
35

C++: Why does std::system("exit 1") return 256?

1 Answers1