bind_port.c

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <arpa/inet.h>

#define BAIL(ret, msg) do { if ((ret) < 0) { perror(msg); exit(1); } } while (0)

int main() {
    int i, fd, ret;
    struct sockaddr_in addr_in, addr_out;
    socklen_t len;

    for (i = 0; i < 10; i++) {
        memset(&addr_in, 0, sizeof(addr_in));
        memset(&addr_out, 0, sizeof(addr_out));

        addr_in.sin_family      = AF_INET;
        addr_in.sin_port        = 0;
        addr_in.sin_addr.s_addr = inet_addr("127.0.0.1");

        fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
        BAIL(fd, "unable to create socket");

        ret = bind(fd, (struct sockaddr *)&addr_in, sizeof(addr_in));
        BAIL(ret, "unable to bind socket");

        len = sizeof(addr_out);
        ret = getsockname(fd, (struct sockaddr *)&addr_out, &len);
        assert(len == sizeof(addr_out));
        BAIL(ret, "getsockname failed");

        ret = close(fd);
        BAIL(ret, "close failed");

        printf("fd %2d, port %d\n", fd, addr_out.sin_port);
    }

    return 0;
}