process communication with signls - c

Question

I am trying to write a C program which has some number of processes. One of them sends a random signal in the range SIGRTMIN and SIGRTMAX to all other processes but I want this signal will be ignored in main process.I used the global variable to have randomized signal to ignore with SIG_IGN. It looks It is not helping because the main stops with real-time signal when wants to ignore the first randomized signal.
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <time.h>
volatile sig_atomic_t disarming_signal = 0;
void disarming_handler (int sig) {
disarming_signal = sig;
fprintf(stderr,"signal %d is handeled", disarming_signal);
}
int rand_range(int min_n, int max_n){
int rand_n = rand() % (max_n - min_n) + min_n;
return rand_n;
}
int sethandler (void (*f)(int), int sigNo) {
struct sigaction act;
memset(&act, 0, sizeof(struct sigaction));
act.sa_handler = f;
if (-1==sigaction(sigNo, &act, NULL))
return -1;
return 0;
}
void sigchld_handler(int sig){
pid_t pid;
for(;;){
pid=waitpid(0, NULL, WNOHANG);
if(pid==0) return;
if(pid<=0) {
if(errno==ECHILD) return;
perror("waitpid:");
exit(EXIT_FAILURE);
}
}
}
void usage(){
fprintf(stderr,"USAGE: sappartherroryst n\n");
fprintf(stderr,"n - number of Therrorysts\n");
}
void therroryst_work(){
int s,k,t;
srand(getpid());
s = rand_range(SIGRTMIN, SIGRTMAX);
t = rand_range(10, 20);
k = t;
if(sethandler(disarming_handler, s)){
perror("Seting therroryst handeler");
exit(EXIT_FAILURE);
}
fprintf(stderr, "[%d] I am therroryst. My disarming signal is [%d]. I will wait [%d] Sec.\n", getpid(), s, t);
while(k>0) {
k=sleep(k);
if(disarming_signal == s){
fprintf(stderr, "I got signal [%d]\n.",disarming_signal);
return ;
}
}
fprintf(stderr, "[%d] KABOOM\n",getpid());
exit(EXIT_SUCCESS);
}
void create_therrorysts(int n){
while(n-->0){
switch(fork()) {
case 0:
therroryst_work();
exit(EXIT_SUCCESS);
case -1:
perror("Fork():");
exit(EXIT_FAILURE);
}
}
}
void sapper_work(){
int sig_dis, i;
struct timespec t, tn = {1,0};
fprintf(stderr,"[%d] I am sapper.\n", getpid());
for(i=0;i<10;i++){
for(t=tn;nanosleep(&t,&t););
sig_dis = rand_range(SIGRTMIN, SIGRTMAX);
if(kill(0, sig_dis)<0){
perror("Disarming_send\n");
exit(EXIT_FAILURE);
}
fprintf(stderr,"I sended signal [%d].\n",sig_dis);
disarming_signal = sig_dis;
}
fprintf(stderr, "end of sending");
exit(EXIT_SUCCESS);
}
void create_sapper(){
switch(fork()) {
case 0:
sapper_work();
exit(EXIT_SUCCESS);
case -1:
perror("Fork():");
exit(EXIT_FAILURE);
}
}
int main(int argc, char** argv){
int n;
pid_t pid;
if(argc != 2){
usage();
return EXIT_FAILURE;
}
n = atoi(argv[1]);
if(n <= 0){
usage();
return EXIT_FAILURE;
}
if(sethandler(sigchld_handler, SIGCHLD)) {
perror("Seting parent SIGCHLD:");
exit(EXIT_FAILURE);
}
create_therrorysts(n);
create_sapper();
sleep(5);
for(;;) {
if(sethandler(SIG_IGN, disarming_signal)){
perror("Seting parent disarming111");
exit(EXIT_FAILURE);
}
}
for(;;){
pid=wait(NULL);
if(pid<0)
switch (errno){
case ECHILD:
return EXIT_SUCCESS;
case EINTR:
continue;
default:
perror("wait:");
exit(EXIT_FAILURE);
}
}
return EXIT_SUCCESS;
}

Answer 1

You have sleep(5) after the create_sapper and before sethandler(IGN). That means it's very likely that the signal is sent before your main process has ignored it.
EDIT: Adding comment from Jonathan Leffler into this answer as it is equally (or even more) important:
There's also a problem with setting the signal handler even if you put the sleep() after that loop - the parent doesn't get to see what the child chooses as disarming_signal.