Signal handling - c

Question

I'm currently coding a function that execute external command for an assignment. Here's what I've done, the problem is that the program is taking too long and is interrupted by a SIGALRM.
Thanks for any help!
volatile sig_atomic_t sig = 0;
void ext(int signum){
if(signum==SIGINT || signum==SIGTERM) sig=1;
}
int extern(char **line){
pid_t p;
int status;
struct sigaction as = {0};
as.sa_handler=ext;
if (sigaction(SIGINT, &as, 0)==-1||sigaction(SIGTERM, &as, 0)==-1){
perror("sigaction");
exit(1);
}
switch(p=fork()){
case -1: perror("fork"); exit(1);
case 0 : if(execvp(line[0], line)<0) {perror("exec"); exit(1);} break;
default : //EDIT
if(waitpid(p, &status, 0)>=0){
if (WIFEXITED(status)) return WEXITSTATUS(status);
} else {
perror("wait");
exit(1);
}
break;
}
return 0;
}

Answer 1

Signal dispositions are not carried across a call to execve. They will be reset as soon as execvp executes.
Even if the signal arrives in the child before execpv executes, the parent and the child process have their own copy of
volatile sig_atomic_t sig = 0;
The signal handler in the child changing this value would not cause the parent's copy to change.
Establishing a signal handler is not the correct approach to take here.
Instead, your approach of using waitpid is the correct one, but alongside checking for a normal termination of the program with WIFEXITED(status), you should include another branch that checks WIFSIGNALED(status), which will be true if the child process terminated due to a signal.
WTERMSIG(status) is used to determine which signal terminated the child process.
Here is a general example where the child process randomly exits successfully, or otherwise raises a signal where the default disposition is to terminate the program:
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>
int main(void)
{
pid_t child = fork();
if (-1 == child) {
perror("fork");
return EXIT_FAILURE;
}
if (0 == child) {
/* randomly signal or return successfully */
srand((unsigned) time(NULL));
switch (rand() % 4) {
case 1: raise(SIGINT); break;
case 2: raise(SIGTERM); break;
case 3: raise(SIGKILL); break;
}
return EXIT_SUCCESS;
}
int status;
if (-1 == waitpid(child, &status, 0)) {
perror("wait");
return EXIT_FAILURE;
}
if (WIFSIGNALED(status)) {
int sig = WTERMSIG(status);
if (SIGINT == sig || SIGTERM == sig)
printf("Child <%ld> exited by signal SIGINT or SIGTERM.\n", (long) child);
else
printf("Child <%ld> exited by signal #%d.\n", (long) child, sig);
} else if (WIFEXITED(status)) {
printf("Child <%ld> exited normally with status %d.\n",
(long) child,
WEXITSTATUS(status));
}
}
Output from running this program a few times:
Child <41268> exited by signal SIGINT or SIGTERM.
Child <41272> exited by signal SIGINT or SIGTERM.
Child <41276> exited by signal #9.
Child <41280> exited normally with status 0.