how to use wait in C

The wait system-call puts the process to sleep and waits for a child-process to end. It then fills in the argument with the exit code of the child-process (if the argument is not NULL).

So if in the parent process you have

int status;
if (wait(&status) >= 0)
{
    if (WEXITED(status))
    {
        /* Child process exited normally, through `return` or `exit` */
        printf("Child process exited with %d status\n", WEXITSTATUS(status));
    }
}

And in the child process you do e.g. exit(1), then the above code will print

Child process exited with 1 status

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Also note that it’s important to wait for all child processes. Child processes that you don’t wait for will be in a so-called zombie state while the parent process is still running, and once the parent process exits the child processes will be orphaned and made children of process 1.