How to sync two process (child and parent) with signals in C? - c

Question

I want use signal (SIGUSR1) to sync process in C. I want that parent process wainting for a signal, when receive this signal, send the same signal to child process. I wrote a short paragraph to stimulate reasoning, but it does not go away.
#include <sys/types.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
void trataSIGUSR1(int sigNum) {
printf("SIGUSR1\n");
}
int main(void) {
pid_t pid;
struct sigaction sa;
pid = fork();
if (pid == 0) {
struct sigaction sa = {0};
sa.sa_handler = trataSIGUSR1;
sa.sa_flags = 0;
sigemptyset(&sa.sa_mask);
sigaction(SIGUSR1,&sa,NULL);
pause();
printf("This never execute");
} else {
printf("I'am father: %d!\n",getppid());
kill(0,SIGUSR1);
pause();
}
}
OUTPUT
I'am father: 12082!
User defined signal 1: 30

Answer 1

Simple hint is use pause() and kill() i.e pause() is for block the process execution until any signal received, once signal received then do_something() and kill() is for sending the SIGUSR1 signal.
Also when you use pause(), it will suspend the process until it received any signal and for that signal default action should be user defined ISR. from manual page of pause()
RETURN VALUE
pause() returns only when a signal was caught and the signal-catching function returned.
In this case pause() returns -1, and errno is set to EINTR.
Here is the sample required example code
//int nSIGINT = 0; /* declare variable of type volatile sigatomic_t */
volatile sigatomic_t nSIGINT;
void trataSIGINT(int sigNum) {
nSIGINT = 1;/* set the flag as needed */
}
int main(void ){
int pid;
pid=fork();/* create child process */
if(pid==0) {
//signal(SIGUSR1,trataSIGINT);/* instead of signal() use sigaction */
struct sigaction sa = {0}; /* initialize sa or fill all its members*/
sa.sa_handler = trataSIGINT;/* set the handler to trataSIGINT*/
sa.sa_flags = 0;
sigemptyset(&sa.sa_mask);
sigaction(SIGUSR1,&sa,NULL); /* when child received SIGUSR1, trataSIGINT gets called */
pause(); /* wait until any signal received */
/* do_something_child() code, this you want to run only after receiving signal */
}
else {
/* do_something_parent() */
printf("parent about to send user signal to child\n");
kill(pid,SIGUSR1); /*send SIGUSR1 to child */
wait(0); /* wait till child completes */
}
return 0;
}
Side note, for setting a flag in trataSIGINT() when SIGUSR1 received, instead of declaring int nSIGINT = 0; declare flag variable as type of volatile sigatomic_t type.
From ISO/IEC 9899:2011 §7.14.1.1 The signal function
¶5 If the signal occurs other than as the result of calling the abort or
raise function, the behavior is undefined if the signal handler
refers to any object with static or thread storage duration that is
not a lock-free atomic object other than by assigning a value to an
object declared as volatile sig_atomic_t, or the signal handler
calls any function in the standard library other than the abort
function, the _Exit function, the quick_exit function, or the
signal function with the first argument equal to the signal number
corresponding to the signal that caused the invocation of the handler.
Furthermore, if such a call to the signal function results in a
SIG_ERR return, the value of errno is indeterminate.252)
252) If any signal is generated by an asynchronous signal handler, the
behavior is undefined.