From Forum Nokia Wiki

The listen method listens for connections on a socket.

int listen (int s, int backlog);

The s argument is a socket and the backlog argument defines the maximum length the queue of pending connections may grow to. The real maximum queue length will be 1.5 times more than the value specified in the backlog argument.

Following is the usage of listen function:

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void listen_example()
{
   int sock_fd;
   int newsock_fd;
   struct sockaddr_in addr;
   struct sockaddr_in ss;
   struct sockaddr_in new_socket;
   unsigned int len;
   unsigned int addr_len;
   
   sock_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
       
   addr.sin_family = AF_INET;
   addr.sin_addr.s_addr = htonl(INADDR_ANY);
   addr.sin_port = htons(5000);
   bind(sock_fd,(struct sockaddr*)&addr,sizeof(addr));
   listen(sock_fd,1);
   close(sock_fd);
}

Wiki Links

Open C Sockets Overview