How to send a signal around specific processes? - c

Question

I'm trying to send a signal around circle of processes for a certain amount of times. my first argument represents the number of processes I wish to create. my second one is just a place holder I am currently initiating to be 0. My third is the number of time I want to pass this signal around. I have designed the processes to have a relationship as such: Parent->child1,
child1->child2, child2->child3.... and so on. I'm just figuring out C and I'm confused to why my code is stopping midway. It runs for an iteration or two and then stalls out. Can someone explain why?
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
int startProcess;
int N;
int numOfCycles;
sigset_t killSet;
void myHandler1 () {
if(N >= 2 && numOfCycles > 0) {
printf("N=%d, numOfCycles=%d, Signal caught. PID = %d\n",N,numOfCycles,getpid());
numOfCycles--;
kill((getpid()+1),SIGUSR1);
}
else if(N >= 2 && numOfCycles == 0) {
exit(1);
}
else if(N == 1 && numOfCycles > 0) {
printf("N=%d, numOfCycles=%d, Signal caught. PID = %d\n",N,numOfCycles,getpid());
numOfCycles--;
kill(startProcess,SIGUSR1);
}
else if(N == 1 && numOfCycles == 0) {
exit(1);
}
else {
printf("Cycle Complete\n");
exit(1);
}
}
void main(int arg, char ** argv) {
struct sigaction temp, vitas, arctic;
sigemptyset(&killSet);
sigaddset(&killSet,SIGUSR1);
N = atoi(argv[1]);
pid_t process1;
startProcess = atoi(argv[2]);
numOfCycles = atoi(argv[3]);
temp.sa_handler = myHandler1;
temp.sa_flags = SA_RESTART;
// vitas.sa_handler = myHandler2;
// vitas.sa_flags = SA_NODEFER;
//
// arctic.sa_handler = myHandler3;
// arctic.sa_flags = SA_RESTART;
sigaction(SIGUSR1, &temp, NULL);
if (N > 1 ) {
process1 = fork();
if(process1 == 0) {
if(N > 2) {
printf("I am a child with PID=%d, PPID=%d, N =%d\n",getpid(),getppid(),N);
}
N--;
char narg = N+'0';
char *pnarg = &narg;
if(startProcess == 0) {
char nstartProcess[6];
startProcess=getppid();
sprintf(nstartProcess,"%d",startProcess);
char *pstartProcess = &nstartProcess[0];
execl("circle",argv[0],pnarg,pstartProcess,argv[3],NULL);
}
else{
if(N == 1){
printf("I am the final child with PID=%d, PPID=%d, N =%d, startProcess=%d\n",getpid(),getppid(),N,startProcess);
printf("\nSignal Passing start\n\n");
kill(startProcess, SIGUSR1);
while(1) {
sigsuspend(&killSet);
}
}
else {
execl("circle",argv[0],pnarg,argv[2],argv[3],NULL);
}
}
}
else {
printf("I am a parent with PID=%d, PPID=%d, N =%d, startProcess=%d\n",getpid(),getppid(),N,startProcess);
wait(NULL);
while(1) {
sigsuspend(&killSet);
}
}
}
}

Answer 1

stalls out. Can someone explain why?
The final child is not woken up from sigsuspend(&killSet) because you did sigaddset(&killSet,SIGUSR1); - you seem to have thought you have to add the signal to be waited for to the set, but on the contrary the signals in the given set are blocked from delivery. So just drop the sigaddset call.

Answer 2

To add to #Armali and as the sigset was not the only point to consider, it's seems more simple to provide an implementation with a far more simple handler and which avoid fork+exec, you'll found explanations in the comments. I'm not sur i'have understood what is your intent, from the comments i guess that you are looking to launch N processus with the last one sending cycles time SIGUSR1 to the parent, each child processus sending SIGUSR1 to their parent on receipt. Correct me if i'm wrong.
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
int startProcess;
int N;
int numOfCycles;
sigset_t killSet;
// Two counters for the signal handler
int numSignals=0;
int numSignalsTotal=0;
// A simple handler that do nothing else that incrementing values in the process.
// To use a SIGINFO handler : void myHandler1 (int signal, siginfo_t *info, void *ucontext) {
void myHandler1 (int signal) {
numSignals++;
numSignalsTotal++;
}
void main(int arg, char ** argv) {
struct sigaction temp;
// As Armali pointed that, here you should not add SIGUSR1 to the sigset a you want
// to suspend your process and waiting for it.
sigemptyset(&killSet);
N = atoi(argv[1]);
pid_t process1 = 0;
startProcess = atoi(argv[2]);
numOfCycles = atoi(argv[3]);
// A more complete initialization here, you need to include SIGUSR1 here to associate it
// to your handler. It seems you doesn't need specific flag like SA_RESTART for it here,
// as the handler code doesn't require that.
temp.sa_handler = myHandler1;
sigemptyset(&temp.sa_mask);
sigaddset(&temp.sa_mask,SIGUSR1);
temp.sa_flags = 0;
temp.sa_restorer = NULL;
// Always check that your function calls return OK. Always.
if (sigaction(SIGUSR1, &temp, NULL) != 0) {
perror("ERR: Failed to set handler ...");
exit(1);
}
// Instead of a mix of fork + exec, it is more simple to keep one process and fork it for each new process.
// Here, we fork at least once, the parent process will alaways be "PARENT" and have a 0 startProcess,
// the child processes will fork themselves, the parent process is their and the new forked one will be
// the next child.
while (startProcess == 0 || N > 1 ) {
process1 = fork();
// To follow the forks : printf(" Fork pid=%d %s %s %s %s\n", process1, argv[0],argv[1],argv[2],argv[3]);
if(process1 != 0 && startProcess == 0) {
// If fork() gives us a pid and we have startProcess 0, then we are the "PARENT" process.
// The only thing we have to do is waiting for signals from our childs, if there is childs.
printf(" I am a parent with PID=%d, PPID=%d, N =%d, startProcess=%d\n",getpid(),getppid(),N,startProcess);
// If there's no child, we simply quit the loop.
if (N == 1) break;
// For reference, "PARENT" output what is its first child.
printf(" -> child = %d\n" , process1);
// Here, while we have cycles, we wait for signals
while(numOfCycles > 0) {
// If you use a handler, you need to operate synchronously with it, so here we block SIGUSR1 as we will wait for it.
if (sigprocmask(SIG_BLOCK, &temp.sa_mask, NULL) != 0) {
perror("ERR: Failed to block usr1...");
exit(1);
}
// We wait for a SIGUSR1.
sigsuspend(&killSet);
// We have a SIGUSR1, we unblock SIGUSR1 as we will modify numSignals and don't want handler doing that at the same time.
if (sigprocmask(SIG_UNBLOCK, &temp.sa_mask, NULL) != 0) {
perror("ERR: Failed to unblock usr1...");
exit(1);
}
// We output on STDOUT that "PARENT" caught a SIGUSR1, we should have the number of signals received in numSignals
printf(" ===> PARENT : N=%d, numOfCycles=%d, Signal caught. PID = %d, count = %d\n",N,numOfCycles,getpid(), numSignals);
// We substract that from our cycles and reset numSignals
numOfCycles-=numSignals; numSignals=0;
}
// If we are here, no more cycles, so we output on STDOUT a summary of "PARENT" processing
printf(" PARENT : N=%d, PID = %d, total = %d\n",N,getpid(), numSignalsTotal);
// Before leaving, "PARENT" should wait for its potential remaining childs to exit
printf(" PARENT waiting for child to terminate.\n");
if (wait(NULL) == -1) {
perror("ERR: PARENt failed to wait for childs termination...");
exit(1);
}
// We quit the loop, so we exit
break;
} else {
// If only one process (N=1) was asked, nothing more to do for the fork, we quit the loop then exit
if (N == 1 && startProcess == 0) break;
// We get the parent process pid in startProcess, as we are a child process
startProcess = getppid();
// If we are the forked process, we decrement N, it will "our" N as child
if (N > 1 && process1 == 0) {
N--;
}
if(process1 == 0) {
// If we are the forked process, we simply state our identity on STDOUT, the final child should be the one with N=1
if (N > 1) {
printf(" I am a child with PID=%d, PPID=%d, N =%d, startProcess=%d\n",getpid(),getppid(),N,startProcess);
} else {
printf(" I am the final child with PID=%d, PPID=%d, N =%d, startProcess=%d\n",getpid(),getppid(),N,startProcess);
}
if (N == 1) {
// If we are the final child, we simply have to send SIGUSR1 signals to our parent process for the number of cycles,
// after that we simply quit the loop and exit, no more forks are required.
while(numOfCycles > 0) {
printf("\n Cycle %d, first Signal start to %d\n\n", numOfCycles--, startProcess);
if (kill(startProcess, SIGUSR1) != 0) {
perror("ERR: LAST CHILD : Failed to send USR1...");
exit(1);
}
}
break;
}
// Note that if we are NOT the final child, we have to go through the loop to fork a new process for the next child.
} else {
if (N > 1) {
// If we are here, we are the forked process for a child and we are not the final child.
// So we do our payload, waiting for SIGUSR1 signals from our child and sending SIGUSR1 to our parent for the
// required number of cycles.
while(numOfCycles > 0) {
// If you use a handle, you need to operate synchronously with it, so here we block SIGUSR1 as we will wait for it.
if (sigprocmask(SIG_BLOCK, &temp.sa_mask, NULL) != 0) {
perror("ERR: Failed to block usr1...");
exit(1);
}
// We wait for a SIGUSR1.
sigsuspend(&killSet);
// We have a SIGUSR1, we unblock SIGUSR1 as we will modify numSignals and don't want handler doing that at the same time.
if (sigprocmask(SIG_UNBLOCK, &temp.sa_mask, NULL) != 0) {
perror("ERR: Failed to unblock usr1...");
exit(1);
}
// We output on STDOUT that this child caught a SIGUSR1, we should have the number of signals received in numSignals
printf(" ===> CHILD: N=%d, numOfCycles=%d, Signal caught. PID = %d, count = %d\n",N,numOfCycles,getpid(), numSignals);
// For each receveid signals from our child, we send one to our parent
while (numSignals > 0) {
printf("\n Signal Passing start to %d\n\n", startProcess);
if (kill(startProcess, SIGUSR1) != 0) {
perror("ERR: CHILD : Failed to send USR1...");
exit(1);
}
// We decrement accordingly our number of cycles and signals
numOfCycles--;
numSignals--;
}
}
// If we are here, no more cycles, so we output on STDOUT a summary of the child processing
printf(" CHILD : N=%d, PID = %d, total = %d\n",N,getpid(), numSignalsTotal);
// We quit the loop, so we exit, then terminate our child
break;
}
}
}
}
// We should always restore signal handler to default when we exit.
temp.sa_handler = SIG_DFL;
sigemptyset(&temp.sa_mask);
sigaddset(&temp.sa_mask,SIGUSR1);
temp.sa_flags = 0;
temp.sa_restorer = NULL;
// Always check that your function calls return OK. Always.
if (sigaction(SIGUSR1, &temp, NULL) != 0) {
perror("ERR: Failed to restore SIGUSR1 ...");
exit(1);
}
// We output on STDOUT the related PID at each process termination.
printf(" End of PID %d\n",getpid());
}