As far as I understand signals sent to a parent process should not be sent to children. So why does SIGINT reach both the child and the parent in the example below?
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
void sigCatcher( int );
int main ( void ) {
if (signal(SIGINT, sigCatcher) == SIG_ERR) {
fprintf(stderr, "Couldn't register signal handler\n");
exit(1);
}
if(fork() == 0) {
char *argv[] = {"find","/",".",NULL};
execvp("find",argv);
}
for (;;) {
sleep(10);
write(STDOUT_FILENO, "W\n",3);
}
return 0;
}
void sigCatcher( int theSignal ) {
write(STDOUT_FILENO, "C\n",3);
}
If you are sending SIGINT by typing ^-C, the signal is sent to all processes in the foreground processing group. If you use kill -2, it will only go to the parent (or whichever process you indicate.)
Related
I have a parent process that is spawning X number of child processes (player) based off the first argument passed to the program. After each child is spawned it sends them a signal. For now all I want the children to do it print they received the signal and exit, but they don't seem to get the signal from the parent. Are the children not getting the signal or am I handling it wrong?
Parent:
#define _POSIX_SOURCE
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
void reciveCard() {
write(1, "signal recived\n", 15);
exit(0);
}
int main(int argc, char **argv) {
int numPlayers, i;
int *kpids;
numPlayers = atoi(argv[1]);
kpids = malloc(numPlayers * sizeof(int));
signal(SIGUSR1, SIG_IGN);
for(i = 0; i < numPlayers; i++) {
if((kpids[i] = fork()) == 0) {
if(execlp("./player\0", "./player\0", (char *) NULL) == -1) {
printf("error\n");
exit(1);
}
}
}
for(i = 0; i < numPlayers; i++) {
printf("%d\n", kpids[i]);
kill(kpids[i], SIGUSR1);
}
wait(NULL);
return 0;
}
Child:
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
void reciveCard() {
write(1, "signal recived\n", 15);
exit(0);
}
int main() {
signal(SIGUSR1, reciveCard);
while(1) {
sleep(1);
}
return 0;
}
You have a race condition.
The parent program does the fork calls so fast that it falls into the kill loop before any child has a chance to do the execlp.
Thus, the child can't set up the handler fast enough. That is, when the signal comes in, the child is still ignoring it because of the parent's SIG_IGN call that it has inherited.
Now, after the execlp, the child will set up the handler, but the signal has already occurred, so the the handler will never be called.
To see this, add a sleep(1) between the two parent loops and it should work.
I have a program that starts a child (r.out), and that child has a time limit.
I want to know how to stop running r.out after passing the time limit.
I am running the code on Linux.
This is what I have so far:
#include <unistd.h>
#include <signal.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#define TIME_LIMIT 1 //determine time limit
void my_function(); //supposed to include the submitted code
void alarm_handler(int);
int main()
{
if (sigaction(SIGALRM, NULL, NULL) == -1)
err(1, NULL);
signal(SIGALRM, alarm_handler); // assigning an alarm handler for SIGALRM
alarm(TIME_LIMIT); // install an alarm to be fired after TIME_LIMIT
system("./r.out"); //Running the file
alarm(0);
return 0;
}
void alarm_handler(int sig)
{
printf("%s" , "Time limit exceeded");
//Here i want a code to stop the r.out file
}
In order to kill the child, you need to know its pid. You can get it if you start the program with fork and exec instead of system.
In addition to a signal handler for SIGALRM, set up one for SIGCHLD (received when a child process finishes) as well. After calling alarm to set the timer, call pause. This function will return when you get either of the two signals.
In the signal handlers you should only set a global flag. Calling printf from within a signal handler can lead to undefined behavior.
After pause returns, check each of the two flags. If the timeout flag is set, you can terminate the child with kill.
In either case, call wait to reap the child process's pid.
#include <unistd.h>
#include <signal.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#define TIME_LIMIT 1 //determine time limit
void alarm_handler(int);
void child_handler(int);
int timeout = 0;
int child_done = 0;
int main()
{
pid_t pid = fork();
if (pid == -1) {
perror("fork failed");
exit(1);
} else if (pid == 0) {
// child process
execl("./r.out","r.out", NULL);
perror("exec failed");
_exit(1);
}
// set up the signal handlers after forking so the child doesn't inherit them
signal(SIGALRM, alarm_handler);
signal(SIGCHLD, child_handler);
alarm(TIME_LIMIT); // install an alarm to be fired after TIME_LIMIT
pause();
if (timeout) {
printf("alarm triggered\n");
int result = waitpid(pid, NULL, WNOHANG);
if (result == 0) {
// child still running, so kill it
printf("killing child\n");
kill(pid, 9);
wait(NULL);
} else {
printf("alarm triggered, but child finished normally\n");
}
} else if (child_done) {
printf("child finished normally\n");
wait(NULL);
}
return 0;
}
void child_handler(int sig)
{
child_done = 1;
}
void alarm_handler(int sig)
{
timeout = 1;
}
I create two children from the parent in the main program. The first and second child executes a program (signalSender) after their creation(alongside with the pid of the other child as an argument). signalSender has signal handler and is used for sending signal between process. The pid of the second child is given as zero as argument when first child executes signalShooter. The pid of the first child is given as argument when the second child executes sigShooter.
1) I want to find the pid of the first child via signal handler after second child sends the signal to first child. I tried to save it to global variable pid_t pid2 but it does not work.
2)I also have to send signal between these two children 100 times but i dont know where to use 'for loop' and ‘wait’ signal.
The main program:
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <signal.h>
int main()
{
pid_t pid1,pid2,wid;
char *my_args[5];
int aInt = 368
char str[15];
pid1 = fork();
if (pid1 < 0)
{
fprintf(stderr, ": fork failed: %s\n", strerror(errno));
exit(1);
}
if(pid1 == 0)
{
my_args[0] = "sigperf1";
my_args[1] = "0";
my_args[2] = NULL;
execv("signalSender",my_args);
fprintf(stderr,"signalSender cannot be executed...");
exit(-1);
}
pid2 = fork();
if(pid2 < 0)
{
fprintf(stderr, ": fork failed: %s\n", strerror(errno));
exit(1);
}
if(pid2 == 0)
{
sprintf(str, "%d", pid1);
my_args[0] = "sigperf1";
my_args[1] = str;
my_args[2] = NULL;
// printf("this is converted = %s\n",my_args[1]);
execv(“signalSender",my_args);
fprintf(stderr,"signalSender cannot be executed...");
exit(-1);
}
wid = wait(NULL);
}
The signalSender:
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <signal.h>
#include <ctype.h>
pid_t pid2;
struct sigaction act;
void sighandler(int signum, siginfo_t *info, void *ptr)
{
printf("Received signal %d\n", signum);
printf("Signal originates from process %lu\n",
(unsigned long)info->si_pid);
pid2 = info->si_pid;
}
int main(int argc,char **argv)
{
memset(&act, 0, sizeof(act));
act.sa_sigaction = sighandler;
act.sa_flags = SA_SIGINFO;
sigaction(SIGUSR1, &act, NULL);
pid_t current, pidOther;
current = getpid();
pidOther = atol(argv[1]);
if(pidOther != 0) // we are in the second child
{
kill(pidOther,SIGUSR1); //sending signal from second child to first
}
if(pidOther == 0) // we are in the first child
{
kill(pid2,SIGUSR1);
}
return 0;
}
You have a synchronization issue here.
Both child processes start at roughly the same time. So you can't predict which one will kill the other first. If the first child runs kill first, it will pass 0 as the pid which will kill every process in the process group. Also, each child process quits immediately after calling kill, so one may exit before the other has a chance to send it a signal.
You need to introduce some type of synchronization method. A simple way to do this is to have the second process sleep briefly before calling kill to give the first process a chance to start up. Similarly, the first process should call pause, which will tell it to wait until it receives a signal.
Once you do that, then each process can call pause and kill in a loop to go back and forth.
if(pidOther != 0) // we are in the second child
{
sleep(1); // wait for first child to start
kill(pidOther,SIGUSR1); //sending signal from second child to first
for (i=0;i<5;i++) {
pause();
kill(pidOther,SIGUSR1);
}
}
if(pidOther == 0) // we are in the first child
{
pause(); // wait until we get a signal from the second child
kill(pid2,SIGUSR1);
for (i=0;i<5;i++) {
pause();
kill(pid2,SIGUSR1);
}
}
#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <limits.h>
#include <unistd.h>
#include <stdlib.h>
#include <pwd.h>
#include <dirent.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
void sig_handler(int signal);
int pid, forkFlag = 0;
int main( int argc, char **argv, char **envp )
{
sigset(SIGINT, sig_handler); //handle ctrl + c
sigignore(SIGTSTP);
sigignore(SIGSTOP);
int ex, rv, status;
forkFlag = 1; //fork is being called
pid = fork();
if(pid == -1){
perror("fork");
exit(2);
}
else if (pid == 0){ //if child process
ex = access(argv[0], X_OK); //check if file is executable
if(ex){
perror("access");
exit(1);
}
else{
rv = execve(argv[0], argv, envp); //run program in child process
if(rv == -1){
perror("execve");
exit(1);
}
}
exit(0); //end child process
}
else{
rv = waitpid(pid, &status, 0); //wait for child
if(rv == -1){
perror("waitpid");
}
if(WEXITSTATUS(status)){ //check status of child if it did ot return 0
printf("The return status of the child was %d\n", WEXITSTATUS(status));
}
}
forkFlag=0;
}
void sig_handler(int signal)
{
if(signal == SIGINT && (pid && forkFlag)){
kill(pid,signal); //send kill to child
}
}
I'm trying to make my program ignore ctrl + C, except when there is a child process running, then it sends the the SIGINT to the child process. However, when I press ctrl + c when the child process is running, waitpid() returns -1 with the error "Interrupted System Call." This makes the child process stop running, but if I use ps, the child process is still there, but now labeled as defunct. I know from printf statements that kill is being calle din the function sig_handler, and that pid and forkFlag are their correct values. Is waitpid() making my program ignore the kill? How do I fix this? I know this code does next to nothing, but it's a small portion of my code (the only part involving fork)
Thanks for any help.
The problem is that the child processes get the same overridden handler for SIGINT. You probably want to reset the signal handler in the child process after the fork, or you might want to install the signal handler in the parent after you've already forked the child, so it doesn't inherit the overriden handler.
I'm having trouble with a process forking exercise. I want to fork a child process and have it hang after announcing it has been forked, and wait for a signal to terminate, after which the parent process must announce it is terminating and then exit.
I can get the processes forked and have the parent wait for the hanging child to be killed by the signal, but it seems to kill the parent as well. I tried killing the child process specifically by its PID, but with no success.
Thanks for any help!
Code:
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sys/types.h>
void catchInt (int signum)
{
printf("\nMy sincerest apologies, master\n");
/*kill(0, SIGINT);*/
exit(0);
}
void ignoreInt (int signum)
{
wait(NULL);
}
int main () {
pid_t pid;
/* fork process */
pid = fork();
if (pid < 0) /* error handler */
{
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0) /* child */
{
printf("Child reporting in\n");
signal(SIGINT, catchInt);
for ( ;; )
pause();
}
else /* parent */
{
/* parent will wait for the child to complete */
signal(SIGINT, ignoreInt);
wait(NULL);
printf("You're welcome\n");
exit(0);
}
}
Even assuming you fix the code so it compiles (you've not defined tempPID), there are problems:
You set the child to go to sleep until a signal arrives.
You set the parent to wait until the child dies.
So, you have a state where neither process is going to do anything more.
You probably need the parent to send a signal to the child:
kill(pid, SIGINT);
It is not clear that you need the parent to set a signal handler.
You probably want the child to set a signal handler.
You probably don't want the infinite loop in the child.
Oh, and void main() is incorrect - int main() or int main(void) or int main(int argc, char **argv) are the approved declarations for main().
And it is tidier if you return a value (0) from main(). The C99 standard does permit you to drop off the end of main() and will treat that as returning zero, but only if the function is properly declared as returning an int.
The header for wait() and relatives in POSIX is <sys/wait.h>.
And, because I'm a sucker, here's code that compiles and might even do what you want:
#include <stdio.h>
#include <signal.h>
#include <unistd.h> /* getpid() */
#include <stdlib.h>
#include <sys/wait.h>
void catchInt(int signum)
{
printf("Child's PID is %d\n", (int)getpid());
printf("My sincerest apologies, master\n");
exit(1);
}
int main()
{
pid_t pid = fork();
if (pid < 0) /* error handler */
{
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0) /* child */
{
printf("Child reporting in\n");
signal(SIGINT, catchInt);
pause();
}
else /* parent */
{
sleep(1);
kill(pid, SIGINT);
wait(NULL);
printf("You're welcome\n");
}
return(0);
}
Just figured out what I was doing wrong, I should have realized SIGINT is sent to every process, and so the parent was simply being sent an unhandled SIGINT, causing it to exit. Thanks for all the help (my apologies on the sloppy coding, I really shouldn't wait until the program is completed to clean that up), the code's been edited above and works as intended.
Thanks again.