How to send signal from a children to another children in C? - c

Question

How can I send a signal form passenger-children to the taxi - children?
I have the following C code:
passengerPid = fork()
if(passengerPid) { //passenger-parents
taxiPid = fork();
if(taxiPid) { //taxi - parents
} else { //taxi - children
}
}
else { //passenger - children
}

Answer 1

How can I send a signal form passenger-children to the taxi - children? yes it's possible but question here is how will you get taxi-children pid in passenger-children process ? once you got pid of that taxi-children, for sending signal to one process from another process you can use kill() function. Use any IPC mechanism to sending pid(any int data) of one process to other process.
Here is my Solution, before this go through man page of wait() & waitpid() :
/**How can I send a signal form passenger-children to the taxi - children? **/
#include<stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main() {
int passengerPid, taxiPid;
passengerPid = fork();
if(passengerPid) { //passenger-parents
taxiPid = fork();
if(taxiPid) {
//taxi - parents
printf("taxi parents or main parent process : pid = %d ppid = %d\n",getpid(),getppid());
/** parents should wait until all children's got finish
use wait() or waitpid() to collect the children's status **/
int s,ret;
while((ret = wait(&s))!=-1)//if there is no further child, wait returns
{
if(ret == passengerPid) {
if(WIFEXITED(s)) {
printf("passenger children removed after sleep, status is : %d\n",WEXITSTATUS(s));
} else if(WIFSIGNALED(s)) {
printf("passenger children removed forcefully because of signal no : %d \n",WTERMSIG(s));
}
} else if(ret == taxiPid) {
if(WIFEXITED(s)) {
printf("taxi children removed after sleep, status is : %d\n",WEXITSTATUS(s));
} else if(WIFSIGNALED(s)) {
printf("taxi children removed forcefully because of signal no : %d \n",WTERMSIG(s));
}
}
}
printf("all done !! parents is also going to complete \n");
}
else {
//for sending pid of this process
#if 0
int fd = open("data",O_WRONLY | 0664);
if(fd == -1) {
perror("open");
return 0;
}
int w_pid = getpid();
write(fd,&w_pid,4);
close(fd);
#endif
//taxi - children
printf("taxi children : pid = %d ppid = %d\n",getpid(),getppid());
sleep(10);//random delay to observe whether this process got any signal or not ?
exit(1);//sends it's exit status to it's parent process whether it's terminated normally or by any external signal
}
}
else {
//passenger - children
printf("passenger children : pid = %d ppid = %d\n",getpid(),getppid());
printf("sending signal from passenger children to taxi children :\n");
sleep(5);
int pid;
#if 0
int fd = open("data",O_RDONLY | 0664);
if(fd == -1) {
perror("open");
return 0;
}
read(fd,&pid,4);
close(fd);
#endif
printf("pid of child received : %d \n",pid);
kill(pid,SIGKILL);//sending signal no 2 to taxi child, bydefault action of SIGINT will be performed
perror("kill");
printf("passenger children dies after sending signal & completion of delay\n");
exit(0);
}
}
how above code works is explained in comments itself. usage of macro(#if 0) part in both process is for sending pid of that taxi-child process to passenger-child process. Enable the macro for observing the output.
I hope it help's.