Why do my program behave unexpectedly when I use sigaction? - c

Question

I'm writing my own simple shell as an exercise. I need to register to the SIGCHLD signal in order to handle zombie-processes. For some reason, when I add the handler using sigaction the program exits, and I don't understand why.
You can see in main() that we exit if process_arglist() returned 0 but I return 1 and I don't see how the signal handling could affect that.
Here's my code. It should handle a command ending with an & (we fork() and use execvp in the child code).
For example: ping 127.0.0.1 -c 5 &.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <pthread.h>
#include <signal.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
void sigchld_handler(int signal) {
int my_errno = errno;
while (waitpid(-1, 0, WNOHANG) > 0); // WNOHANG so handler will be non-blocking.
errno = my_errno;
}
int handle_background_command(int count, char** arglist) {
pid_t pid;
arglist[count - 1] = NULL; // remove "&" from arglist
//Handle SIGCHLD signal
struct sigaction sa, sa_dft;
sa.sa_handler = sigchld_handler;
sa.sa_flags = SA_NOCLDSTOP;
if (sigaction(SIGCHLD, &sa, &sa_dft) == -1) {
perror("error when trying to set signal action");
exit(-1);
}
if((pid = fork()) == -1) {
perror("error when trying to fork() from handle_background_command()");
exit(1);
}
if(pid == 0) {
// Child code
sigaction(SIGCHLD, &sa_dft, NULL);
if(execvp(arglist[0], arglist) == -1) {
perror("error when trying to execvp() from handle_background_command()");
exit(1);
}
}
// Parent code
return 1;
}
int process_arglist(int count, char** arglist)
{
return handle_background_command(count, arglist);
}
int main(void)
{
while (1)
{
char** arglist = NULL;
char* line = NULL;
size_t size;
int count = 0;
if (getline(&line, &size, stdin) == -1) {
printf("out!");
break;
}
arglist = (char**) malloc(sizeof(char*));
if (arglist == NULL) {
printf("malloc failed: %s\n", strerror(errno));
exit(-1);
}
arglist[0] = strtok(line, " \t\n");
while (arglist[count] != NULL) {
++count;
arglist = (char**) realloc(arglist, sizeof(char*) * (count + 1));
if (arglist == NULL) {
printf("realloc failed: %s\n", strerror(errno));
exit(-1);
}
arglist[count] = strtok(NULL, " \t\n");
}
if (count != 0) {
int result = process_arglist(count, arglist);
printf("result = %d\n", result);
if (!result) {
free(line);
free(arglist);
printf("out\n");
break;
}
}
free(line);
free(arglist);
}
pthread_exit(NULL);
return 0;
}
Again, if I get rid of the signal handling code then it works.
What's the reason?
EDIT
Here's the output (last rows) of strace utility:
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=2818, si_status=0, si_utime=0, si_stime=0} ---
wait4(-1, NULL, WNOHANG, NULL) = 2818
wait4(-1, NULL, WNOHANG, NULL) = -1 ECHILD (No child processes)
rt_sigreturn() = -1 EINTR (Interrupted system call)
write(1, "out!", 4out!) = 4
exit_group(0) = ?
+++ exited with 0 +++

Answer 1

Your program exits with EINTR (interrupted system call) from getline function (getline is interrupted by signal handler).
Check this: How to handle EINTR (interrupted System Call)