UID of unix domain socket

udsuid.c

#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/un.h>
#include <err.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

const char SOCK_PATH[] = "/tmp/foo-socket";

int main(int argc, char *argv[])
{
    int ss = socket(AF_LOCAL, SOCK_STREAM, 0);
    if (ss < 0) {
        err(1, "server socket");
    }
    struct sockaddr_un sun;
    sun.sun_len = sizeof(sun);
    sun.sun_family = AF_LOCAL;
    strcpy(sun.sun_path, SOCK_PATH);
    if (bind(ss, (const struct sockaddr *) &sun, sizeof(sun)) < 0) {
        err(1, "bind");
    }
    if (listen(ss, 1) < 0) {
        err(1, "listen");
    }
    int cs = socket(AF_LOCAL, SOCK_STREAM, 0);
    if (cs < 0) {
        err(1, "client socket");
    }
    if (connect(cs, (const struct sockaddr *) &sun, sizeof(sun)) < 0) {
        err(1, "connect");
    }
    struct stat st;
    if (fstat(cs, &st) < 0) {
        err(1, "fstat");
    }
    printf("uid = %d\n", st.st_uid);
    close(cs);
    close(ss);
    unlink(SOCK_PATH);
    return 0;
}