how do I find in C that a port is free to use?

Question

The OS is Linux. I have a server process that can change its port realtime. However I would like to know in advance if a port is free before binding.

Scenario: Server bi

Answer 1

I struggled with this myself and have slightly modified your code.

The solution is to set serv_addr.sin_port = 0 (auto assign port).

Note In the bind() and the getsockname() lines there are unclean casts from sockaddr_in to sockaddr. I have seen it done many places and I'm looking for a safer solution.

#include 
#include 
#include 

// ... snip ...

int sock = socket(AF_INET, SOCK_STREAM, 0);
if(sock < 0) {
    printf("socket error\n");
    return;
}
printf("Opened %d\n", sock);

struct sockaddr_in serv_addr;
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = 0;
if (bind(sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
    if(errno == EADDRINUSE) {
        printf("the port is not available. already to other process\n");
        return;
    } else {
        printf("could not bind to process (%d) %s\n", errno, strerror(errno));
        return;
    }
}

socklen_t len = sizeof(serv_addr);
if (getsockname(sock, (struct sockaddr *)&serv_addr, &len) == -1) {
    perror("getsockname");
    return;
}

printf("port number %d\n", ntohs(serv_addr.sin_port));


if (close (sock) < 0 ) {
    printf("did not close: %s\n", strerror(errno));
    return;
}

Program output

Opened 4
port number 59081