output

forked child with pid 7505 master fd is 3, 1 outstanding
forked child with pid 7506 master fd is 4, 2 outstanding
forked child with pid 7507 master fd is 5, 3 outstanding
forked child with pid 7508 master fd is 6, 4 outstanding
forked child with pid 7509 master fd is 7, 5 outstanding
forked child with pid 7510 master fd is 8, 6 outstanding
forked child with pid 7511 master fd is 9, 7 outstanding
forked child with pid 7512 master fd is 10, 8 outstanding
forked child with pid 7513 master fd is 11, 9 outstanding
forked child with pid 7514 master fd is 12, 10 outstanding

sigchld_test.c

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/select.h>
#include <strings.h>
#include <unistd.h>
#include <util.h>


volatile sig_atomic_t got_sigchld = 0;

void sigchld_handler(int sig) {
	got_sigchld = 1;
}


struct childinfo {
	pid_t pid;
	int amaster;
};


int main(int argc, char **argv) {
	sigset_t sigmask_block;
	sigset_t sigmask_empty;

	fd_set to_read;
	int fd_max;

	int outstanding_children = 0;
	int children_to_create = 10;
	struct childinfo kids[10];
	bzero(&kids, sizeof(kids));

	int amaster = -1;

	pid_t pid;

	sigemptyset(&sigmask_empty);
	sigemptyset(&sigmask_block);
	sigaddset(&sigmask_block, SIGCHLD);

	sigprocmask(SIG_BLOCK, &sigmask_block, NULL);

	struct sigaction sa;
	bzero(&sa, sizeof(sa));
	sa.sa_flags = 0;
	sigemptyset(&sa.sa_mask);
	sa.sa_handler = sigchld_handler;
	sigaction(SIGCHLD, &sa, NULL);

	while (children_to_create-- > 0) {
		if (!(pid = forkpty(&amaster, NULL, NULL, NULL))) {
			return 0;
		} else if (pid < 0) {
			perror("fork");
		} else {
			outstanding_children++;
			printf("forked child with pid %d master fd is %d, %d outstanding\n", pid, amaster, outstanding_children);
			kids[children_to_create].pid = pid;
			kids[children_to_create].amaster = amaster;
		}
	}

	do {
		FD_ZERO(&to_read);
		fd_max = 0;
		for (int i = 0; i < 10; ++i) {
			if (kids[i].pid) {
				FD_SET(kids[i].amaster, &to_read);
				if (kids[i].amaster >= fd_max) {
					fd_max = kids[i].amaster + 1;
				}
			}
		}

		pselect(fd_max, &to_read, NULL, NULL, NULL, &sigmask_empty);

		if (got_sigchld) {
			printf("got_sigchld\n");
			while ((pid = waitpid(-1, NULL, WNOHANG)) > 0) {
				outstanding_children--;
				printf("waited child %d, %d outstanding\n", pid, outstanding_children);
				for (int i = 0; i < 10; ++i) {
					if (kids[i].pid == pid) {
						close(kids[i].amaster);
						break;
					}
				}
			}
			if (pid < 0) {
				perror("waitpid");
			}
			got_sigchld = 0;
		}

	} while(outstanding_children);
}