issue-47955.c

#include <sys/time.h>
#include <sys/resource.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <resolv.h>
#include <netdb.h>

void set_max_fds(int count) {
	struct rlimit rlim;
	getrlimit(RLIMIT_NOFILE, &rlim);
	rlim.rlim_cur = count;
	setrlimit(RLIMIT_NOFILE, &rlim);
}

void try_resolve(char *msg) {
	struct addrinfo *res;
	printf("%s: resolving localhost:1234: ", msg);
	int r = getaddrinfo("localhost", "1234", NULL, &res);
	if (r != 0) {
		printf("%s\n", gai_strerror(r));
	} else {
		printf("ok\n");
	}
}

int main() {
	try_resolve("initial");
	set_max_fds(3);
	try_resolve("with too many fds open");
	set_max_fds(128);
  
  // uncomment this hack to make it work
  // h_errno = 0;
  
	try_resolve("with fds available again");

	return 0;
}

issue-47955.rs

use std::io;
use std::net::ToSocketAddrs;
use libc::{RLIMIT_NOFILE, getrlimit, rlimit, setrlimit};

fn set_max_fds(count: usize) -> io::Result<()> {
    let mut rlim = rlimit{ rlim_cur: 0, rlim_max: 0 };
    unsafe {
        if getrlimit(RLIMIT_NOFILE, &mut rlim as *mut rlimit) != 0 {
            return Err(io::Error::last_os_error());
        }
        rlim.rlim_cur = count as u64;
        if setrlimit(RLIMIT_NOFILE, &rlim as *const rlimit) != 0 {
            Err(io::Error::last_os_error())
        } else {
            Ok(())
        }
    }
}

fn try_resolve(msg: &str) {
    let addr = "localhost:1234";
    println!("{}: resolving {}: {:?}", msg, addr, addr.to_socket_addrs());
}

fn main() -> Result<(), Box<std::error::Error>> {

    try_resolve("initial");
    set_max_fds(3)?;
    try_resolve("with too many fds open");
    set_max_fds(128)?;

    // uncomment this hack to make it work
    // extern { fn __h_errno_location() -> *mut i32; }
    // unsafe { *__h_errno_location() = 0 }

    try_resolve("with fds available again");

    Ok(())
}