Why do my program behave unexpectedly when I use sigaction?

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)