Open a server socket at which TCP connections may be accepted.

open_server_socket.c

#include <stdio.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <arpa/inet.h>

/**
 * Returns a socket listening at the specified port. Up to "backlog" connection
 * requests will be queued before further requests are refused.
 * 
 * On error, -1 is returned.
 */
int open_server_socket(unsigned short port, int backlog)
{
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0)
    {
        fprintf(stderr, "Could not open socket. %s.\n", strerror(errno));
        return -1;
    }

    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &(int){1}, sizeof(int)) < 0)
    {
        fprintf(stderr, "Could not set SO_REUSEADDR. %s.\n", strerror(errno));
        close(sockfd);
        return -1;
    }

    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    addr.sin_port = htons(port);

    if (bind(sockfd, (const struct sockaddr *)&addr, sizeof(addr)) <0)
    {
        fprintf(stderr, "Could not bind to port %hu. %s.\n", port, strerror(errno));
        close(sockfd);
        return -1;
    }

    if (listen(sockfd, backlog) < 0)
    {
        fprintf(stderr, "Could not listen on socket. %s.\n", strerror(errno));
        close(sockfd);
        return -1;
    }

    return sockfd;
}