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 binds localhost:5000 and receives a request to bind at localhost:6000. The server has to check if the port is free. This question seeks for answers that provide a routine that checks if a port is free or not.

For the record, I am editing my question with a code snippet that checks if a port is free to use. This does not mean that it will be used. The code below answer to the question "if the port is available right now", it does not use it. Opens a socket, check if bind returns EADDRINUSE and closes the socket.

#include <iostream>
#include <sys/socket.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <errno.h>

int main(int argc, char **argv) {

    struct sockaddr_in serv_addr;
    if( argc < 2 )
        return 0;
    int port = atoi(argv[1]);

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

     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 = htons(port);
     if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {

        if( errno == EADDRINUSE )
        {
            printf("the port is not available. already to other process\n");
        } else {
            printf("could not bind to process (%d) %s\n", errno, strerror(errno));
        }
    }

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

    return 0;


}

Here are some sample runs (partial outputs)

[bash{1051}{51}]:[~/some_sources/checkbind]::./a.out 41067
the port is not available. already to other process
[bash{1052}{52}]:[~/some_sources/checkbind]::./a.out 22
could not bind to process (13) Permission denied
[bash{1053}{53}]:[~/some_sources/checkbind]::./a.out 22000
Opened fd 3

Solution 1

This is an obvious race condition, since other processes on your system might be binding to ports in parallel. So, any solution you find will be imperfect, and you will still need to just write it according to the "try to bind(), if it fails pick a new port number and try again" approach.

Solution 2

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

// ... 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

Solution 3

If your server was told what port to use, just bind() it. Seriously.

Sure, you could parse /proc/net/tcp and see if the port's in use. But then what? You still need to call bind() now that you know your port is free, and it'll tell you if the port was free then anyway, and so there was no point in groveling through /proc/net/tcp and doing all that (slow!) string production and parsing and extra kernel trips through not-very-well-optimized (read: super slow compared to bind()) diagnostic paths, just to get information that could well be out of date before you even finished parsing it. So just call bind() and be happy.