How to get the first available TCP port to listen to?

Question

I want to attach my service to some TCP port, for testing purposes. Then, I will make calls to this port, make sure the service works, and shut it down. I need to find a way how to generate such a port number, since I can't use a fixed one - tests may run in parallel. Is it possible?

Answer 1

This is how I do it:

#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main() {
    struct sockaddr_in addr;
    socklen_t len = sizeof(addr);
    addr.sin_port = 0;
    int sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock < 0) {
        perror("socket()");
        return -1;
    }
    if (bind(sock, (struct sockaddr*) &addr, sizeof(addr)) != 0) {
        perror("bind()");
        return -1;
    }
    if (getsockname(sock, (struct sockaddr*) &addr, &len) != 0) {
        perror("getsockname()");
        return -1;
    }
    printf("%d\n", addr.sin_port);
    return 0;
}

Then, compile it and run:

cc -o reserve reserve.c && ./reserve

And it outputs the port number, available for binding.

I published a full version of the tool in Github: https://github.com/yegor256/random-tcp-port