在 MacOS 上获取套接字对等 IP 地址的正确方法

Question

I'm trying this simple code, to get the client's IP address. It works nice on FreeBSD, but strangely returns zeroes on MacOS. I'm confused and can't understand what's wrong.

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

int main() {
    int s, c;
    socklen_t len;
    struct sockaddr_in saddr, caddr;

    if ((s = socket(PF_INET, SOCK_STREAM, 0)) == -1) {
        printf("socket()\n"); exit(1);
    }
    
    saddr.sin_family = AF_INET;
    saddr.sin_addr.s_addr = htonl(INADDR_ANY);
    saddr.sin_port = htons(9090);

    if ((bind(s, (struct sockaddr *)&saddr, sizeof(saddr))) != 0) {
        printf("bind()\n"); exit(1);
    }

    if ((listen(s, 5)) != 0) {
        printf("listen()\n"); exit(1);
    }

    if ((c = accept(s, (struct sockaddr *)&caddr, &len)) < 0) {
        printf("accept()\n"); exit(0);
    }

    char ipstr[INET6_ADDRSTRLEN];
    inet_ntop(AF_INET, &caddr.sin_addr, ipstr, len);
    
    printf("Client IP address: [%s:%d]\n", ipstr, ntohs(caddr.sin_port));

    close(c);  close(s);

    return 0;
}

On FreeBSD:

Client IP address: [127.0.0.1:17225]

On MacOS:

Client IP address: [0.0.0.0:0]

Answer 1

You must initialize len before calling accept . It tells accept the size of the structure passed to it:

len = sizeof caddr;
if ((c = accept(s, (struct sockaddr *)&caddr, &len)) < 0) { ... }

---

If it's not initialized it will have an indeterminate (read: garbage) value which could lead to undefined behavior .