Using Signal Handlers to Pause/Resume a Child Process - c

Question

I'm currently trying to experiment with signals in C by using them to control a child process created with the fork() method. Essentially, I have a child process running the "yes" command from the linux terminal (this command just prints "y" and a newline until it is terminated). I want to be able to pause/resume this process with CTRL-Z. This is what i've got right now:
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
pid_t CHILD_PROCESS;
pid_t PARENT_PROCESS;
int isPaused;
void pause_handler(int signo){
if(!isPaused){
printf("Ctrl-Z pressed. Pausing child.\n");
isPaused = 1;
kill(CHILD_PROCESS,SIGSTOP);
}
else if(isPaused){
printf("\nCtrl-Z pressed. Resuming child.\n");
kill(CHILD_PROCESS,SIGCONT);
isPaused = 0;
}
}
int main(int argc, char** argv){
pid_t pid;
PARENT_PROCESS = getpid();
pid = fork();
if(pid == 0){
system("yes");
}
isPaused = 0;
if(pid > 0){
signal(SIGTSTP, SIG_IGN);
signal(SIGSTOP, SIG_IGN);
CHILD_PROCESS = pid;
while(1){
if(signal(SIGTSTP,pause_handler) == SIG_ERR){
printf("Signal Failure");
}
}
}
}
When I run this, I can get "Ctrl-Z pressed. Pausing child." to print to console by pressing CTRL-Z, and I can get "Ctrl-Z pressed. Resuming child." to print to the console by pressing CTRL-Z again. However, it doesn't actually resume printing "y" over and over again. Any ideas as to why the child process isn't resuming?

Answer 1

As it turns out, system has an implicit fork call within it, so the PID that gets stored in CHILD_PROCESS ends up not actually being the child process, and instead an intermediate one.
From man 3 system:
The system() library function uses fork(2) to create a child process
that executes the shell command specified in command using execl(3) as
follows:
execl("/bin/sh", "sh", "-c", command, (char *) 0);
system() returns after the command has been completed.
So, if we replace the system("yes") call with execl("/bin/sh", "sh", "-c", "yes", NULL), then we avoid this extra fork and the program functions as desired.
The only other issue is that, by I comment I found on this post, using printf within a signal handler is undefined behavior. Not an issue to worry about here, but something to keep in mind for future code!