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#include <signal.h>
#include <stdlib.h>
#include <stdio.h>

typedef void (*pfv)(int);

void handler(int signum) {
  pfv old_handler = signal(signum, SIG_DFL);
  if (old_handler == SIG_ERR) {
    perror("SIGINT handler"); /* undefined behavior */
    /* Handle error condition */
  }
}

int main(void) {
  pfv old_handler = signal(SIGINT, handler);
  if (old_handler == SIG_ERR) {
    perror("SIGINT handler");
    /* Handle error condition */
  }

  /* Main code loop. */

  return EXIT_SUCCESS;
}

#include <signal.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>

void reaper(int signum) {
  errno = 0;
  for (;;) {
    int rc = waitpid(-1, NULL, WNOHANG);
    if ( (0 == rc) || (-1 == rc && EINTR != errno)) ){
      break;
    }
  }
  if (ECHILD != errno) {
    /* Handle error */
  }
}

int main(void) {
  struct sigaction act;
  act.sa_handler = reaper;
  act.sa_flags = 0;
  if (sigemptyset(&act.sa_mask) != 0) {
    /* Handle error */
  }
  if (sigaction(SIGCHLD, &act, NULL) != 0) {
    /* Handle error */
  }

  /* ... */

  return EXIT_SUCCESS;
}

#include <signal.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>

void reaper(int signum) {
  int save_errno = errno;
  errno = 0;
  for (;;) {
    int rc = waitpid(-1, NULL, WNOHANG);
    if ( (0 == rc) || (-1 == rc && EINTR != errno)) ){
      break;
    }
  }
  if (ECHILD != errno) {
    /* Handle error */
  }
  errno = save_errno;
}

int main(void) {
  struct sigaction act;
  act.sa_handler = reaper;
  act.sa_flags = 0;
  if (sigemptyset(&act.sa_mask) != 0) {
    /* Handle error */
  }
  if (sigaction(SIGCHLD, &act, NULL) != 0) {
    /* Handle error */
  }

  /* ... */

  return EXIT_SUCCESS;
}