System call kill convert a child to be a zombie process - c

Question

I wrote a code that creates a child with a fork(). The parent of this child should sent a SIGUSR1/2 to its CHILD and the child should answer with a SIGUSR2/1.
This is the code:
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
void HDL_PSIGUSR(int sig) {
printf("Signal 0x%x received.\n\n", sig);
fflush(stdout);
}
void HDL_SSIGUSR(int sig) {
if (sig == SIGUSR1) {
printf("PID %d -> PID %d: 0x%x\n", getpid(), getppid(), SIGUSR2);
kill(getppid(), SIGUSR2);
} else if (sig == SIGUSR2) {
printf("PID %d -> PID %d: 0x%x\n", getpid(), getppid(), SIGUSR1);
kill(getppid(), SIGUSR1);
}
fflush(stdout);
}
void HDL_SSIGINT(int sig) {
kill(getppid(), SIGINT);
}
void son() {
signal(SIGUSR1, HDL_SSIGUSR);
signal(SIGUSR2, HDL_SSIGUSR);
signal(SIGINT, HDL_SSIGINT);
signal(SIGALRM, SIG_IGN);
signal(SIGCHLD, SIG_IGN);
while (true) {
pause();
}
}
int main() {
int* _buf, n, i, t, timer = 0;
pid_t pid;
char buff[1000];
printf("Number of signals to send: ");
scanf("%d", &n);
printf("Interval time: ");
scanf("%d", &t);
printf("Signals to send: ");
_buf = malloc(n * sizeof *_buf);
for (i = 0; i < n; i++) {
scanf("%d", &_buf[i]);
}
fflush(stdout);
if (pid = fork()) {
signal(SIGUSR1, HDL_PSIGUSR);
signal(SIGUSR2, HDL_PSIGUSR);
i = 0;
while (true) {
i %= n;
//sprintf(buff, "kill -USR1 %d", pid);
//system(buff);
kill(pid, SIGUSR1);
sleep(t);
}
} else {
son();
exit(0);
}
waitpid(pid, (int*)0, 0);
return 0;
}
The problem is that if I use the kill() system call, the child process become a zombie process. Instead, if I use the system() system call and calling from there the command kill on CHILD PID, it works! Why?

Answer 1

Thank you guys! The problem was as said by #Jonathan Leffler and #Shawn. It can be solved with a sleep(1); in the parent process before starting to send signals. I modified my code as shown here:
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
bool startProg = false;
void HDL_PSIGUSR(int sig) {
if (!startProg) {
startProg = true;
return;
}
printf("Received signal 0x%x\n", sig);
fflush(stdout);
}
void HDL_SSIGUSR(int sig) {
kill(getppid(), (sig == SIGUSR1) ? SIGUSR2 : SIGUSR1);
}
void HDL_SSIGINT(int sig) {
kill(getppid(), SIGINT);
}
void son() {
signal(SIGUSR1, HDL_SSIGUSR);
signal(SIGUSR2, HDL_SSIGUSR);
signal(SIGINT, HDL_SSIGINT);
signal(SIGALRM, SIG_IGN);
signal(SIGCHLD, SIG_IGN);
kill(getppid(), SIGUSR1);
while (true) {
pause();
}
}
int main() {
int* _buf, n, i, t;
pid_t pid;
printf("Number of signals to send: ");
scanf("%d", &n);
printf("Interval time: ");
scanf("%d", &t);
printf("Signals to send: ");
_buf = malloc(n * sizeof *_buf);
for (i = 0; i < n; i++) {
scanf("%d", &_buf[i]);
}
fflush(stdout);
if (pid = fork()) {
signal(SIGUSR1, HDL_PSIGUSR);
signal(SIGUSR2, HDL_PSIGUSR);
while (!startProg) {
pause();
}
i = 0;
while (true) {
i %= n;
kill(pid, _buf[i++]);
pause();
sleep(t);
}
} else {
son();
exit(0);
}
waitpid(pid, (int*)0, 0);
return 0;
}