How to send signal to a child process created via fork and execl? - process

Question

Assume I have a program foo that simply prints forever but is stopped before then,
int main(int argc, char * argv[])
{
kill(getpid(), SIGSTOP);
while(1) {
printf("foo\n");
}
return 0;
}
And then I have a program bar that forks and runs foo:
int main(int argc, char * argv[])
{
pid_t pid = fork();
if (pid == 0)
{
execl("foo", "foo", NULL);
}
return 0;
}
Now, I want bar to be able to send SIGCONT to foo so it can go on its printing duties, and then later send SIGSTOP again, and even later send SIGCONT again, and so on.
I couldn't figure out how to do that. Any help?
EDIT: I figured out my problem. Apparently, the foo program isn't ready to accept signals immediately when it runs. When I did
sleep(5);
int rc = kill(pid, SIGCONT);
it worked.

Answer 1

pid_t pid = fork();
if (pid == 0)
{
execl("foo", "foo", NULL);
}
int rc = kill(pid, 18);
return 0;

Answer 2

Suggest: don't forget to handle errors codes from system calls! Work without handling of error codes is worse style! This two actions you can do in one program. In this case it's will be more effective:
#include <errno.h>
int main(int argc, char * argv[])
{
int rc = 0;
pid_t pid = fork();
if (pid == 0) /* child */
{
kill(getpid(), SIGSTOP);
if (rc == -1) {
perror("Something wrong while kill() in child");
}
while (1) {
printf("foo\n");
}
} else if (pid > 0) { /* father */
rc = kill(pid, SIGCONT);
if (rc == -1) {
perror("Something wrong while kill() in parent");
}
} else if (pid < 0) {
perror("Something wrong while fork()");
return -1;
}
return 0;
}