How to handle SIGHLD - c

Question

I'm having some troubles using sigchld...
what I want to do is to create a child process with fork and make the child print and sleep a second for a couple of times... during these process I want to send signal to child (SIGSTOP and SIGCONTINUED) and I want the parent to print what the signal was... here's my code:
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/types.h>
void handler (int i) {
int x;
waitpid(-1,&x, 0);
printf("WIFSTOPPED=%d, WIFCONTINUED=%d\n", WIFSTOPPED(x),WIFCONTINUED(x) );
}
int main(){
int x;
int q=fork();
if(q==0){
int i=0;
printf("%d\n",getpid());
while (i<20){
printf("%d\n", i++);
sleep(1);
}
_exit(0);
}
else {
signal(SIGCHLD, handler);
waitpid(-1,&x, 0);
while(WIFEXITED(x)!=1){
waitpid(-1,&x, 0);
sleep(1);
}
exit(0);
}
}
but it doesn't work beacause when I send a SIGSTOP or SIGCONTINUED to the child, the child stop and continue but the parent doesn't print anything
any suggestion?

Answer 1

Your handler shall not call waitpid again and you main while loop is also not correct : again you call waitpid twice the first time. And last, your waitpid call much declare to be interested in status changes (WUNTRACED option).
A much correct code could be :
void handler (int i) { // a handler just handle the fact some signal occured
printf("in handler\n");
}
int main(){
int x;
int q=fork();
if(q==0){
int i=0;
printf("%d\n",getpid());
while (i<20){
printf("%d\n", i++);
sleep(1);
}
_exit(0);
}
else {
signal(SIGCHLD, handler); // catch child status changes
do {
waitpid(-1,&x, WUNTRACED|WCONTINUED); // wait until child terminates or changes its status
if (WIFSTOPPED(x)|WIFCONTINUED(x)) // test what really happens
printf("STOPPED=%d, CONTINUED=%d\n", WIFSTOPPED(x),WIFCONTINUED(x) );
} while(!WIFEXITED(x));
exit(0);
}
}