Background Process not running?

Question

I ran a C program with an infinite while loop in a terminal and observed one of the CPU cores shot to 100% usage. When I turn it into a background process using Ctrl+z, the CPU usage suddenly dipped to < 10%. Using htop, I found that the process was using 0% of the CPU. What has happened to the process?

The program

int main()
{
    int i;
     while( 1 )
    { }
    return 0;
}

Since there is no I/O involved in the program, I made small changes to the program to see if the I/O wait increased the usage, but got the same results (0% usage).

int main()
{
    int i;
     while( i+1 ) // wait to retrieve i
    {
        i = 1; 
    }
    return 0;
}

What has happened to the process ? Is it running?

Answer 1

When you press Ctrl+z in your terminal, a SIGTSTP is sent to the process group of the task running in the foreground.

The default action of SIGSTP is to stop (i.e. suspend) the process:

     18    SIGTSTP      stop process         stop signal generated from keyboard

If you want the process to resume operation in the background, you can use the shell-builtin bg (short for background). If you want the process to resume operation in the foreground, you can use the fg command.