TCP server: spawn arguments with each connection on fds 0, 1

tcpsrv.c

#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <ctype.h>
#include <stdlib.h>
#include <stdio.h>
#include <err.h>

int setreuseaddr (int s, int v) {
	return setsockopt (s, SOL_SOCKET, SO_REUSEADDR, &v, sizeof (int));
}

int main (int argc, char *argu[]) {
	struct sockaddr_in6 a = {
		.sin6_family = AF_INET6,
		.sin6_addr.s6_addr = { 0 },
		.sin6_port = argc < 2 ? errx (1, "no port given"), 0
		                      : htons (strtoul (argu[1], 0, 10)),
		.sin6_scope_id = 0,
		.sin6_flowinfo = 0,
	};
	int s;
	size_t l = sizeof (struct sockaddr_in6);
	
	if (argc < 2) errx (1, "no port given");
	if ((s = socket (AF_INET6, SOCK_STREAM, IPPROTO_TCP)) < 0 ||
	    setreuseaddr (s, 1) < 0 ||
	    bind (s, (struct sockaddr *)&a, l) < 0 ||
	    listen (s, SOMAXCONN) < 0) err (1, "failed");
	
	for (;;) {
		pid_t pid;
		int c, t;
		
		if ((t = accept (s, (struct sockaddr *)&a, &l)) < 0) err (1, "failed to accept");
		
		pid = fork ();
		if (pid < 0) err (1, "failed to fork");
		if (pid > 0) {
			close (t);
			waitpid (pid, &c, 0);
			continue;
		}
		
		{
			char acs[40];
			inet_ntop (AF_INET6, &a.sin6_addr, acs, 40);
			for (int ii = 0; ii < 40; ii++) acs[ii] = toupper (acs[ii]);
			setenv ("REMOTE_ADDR", acs, 1);
		}
		
		dup2 (t, 0);
		dup2 (t, 1);
		
		pid = fork ();
		if (pid < 0) err (1, "failed to fork");
		if (pid > 0) return 0;
		return (execvp (argu[2], argu + 2));
	}
}