This server uses the fork function to create a new process for each client connection, permitting as many clients as resources will allow. It only works on Linux, and is best suited to server computers with 1 or 2 cores.

Notice that when using fork it is necessary to set up a signal handler for SIGCHLD in order to reap zombie processes.

#include <stdio.h>
#include <string.h> /* memset() */
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <netdb.h>

#define PORT    "32001" /* Port to listen on */
#define BACKLOG     10  /* Passed to listen() */

/* Signal handler to reap zombie processes */
static void wait_for_child(int sig)
{
	while (waitpid(-1, NULL, WNOHANG) > 0);
}

void handle(int newsock)
{
	/* recv(), send(), close() */
}

int main(void)
{
    int sock;
	struct sigaction sa;
	struct addrinfo hints, *res;
	int reuseaddr = 1; /* True */

	/* Get the address info */
	memset(&hints, 0, sizeof hints);
	hints.ai_family = AF_INET;
	hints.ai_socktype = SOCK_STREAM;
	if (getaddrinfo(NULL, PORT, &hints, &res) != 0) {
		perror("getaddrinfo");
		return 1;
	}

	/* Create the socket */
    sock = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
	if (sock == -1) {
		perror("socket");
		return 1;
	}

	/* Enable the socket to reuse the address */
	if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &reuseaddr, sizeof(int)) == -1) {
		perror("setsockopt");
		return 1;
	}

	/* Bind to the address */
    if (bind(sock, res->ai_addr, res->ai_addrlen) == -1) {
        perror("bind");
        return 1;
    }

	/* Listen */
    if (listen(sock, BACKLOG) == -1) {
        perror("listen");
        return 1;
    }

	freeaddrinfo(res);

	/* Set up the signal handler */
	sa.sa_handler = wait_for_child;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    if (sigaction(SIGCHLD, &sa, NULL) == -1) {
        perror("sigaction");
		return 1;
    }

	/* Main loop */
	while (1) {
		struct sockaddr_in their_addr;
		socklen_t size = sizeof(struct sockaddr_in);
		int newsock = accept(sock, (struct sockaddr*)&their_addr, &size);
		int pid;

		if (newsock == -1) {
			perror("accept");
			return 0;
		}

		printf("Got a connection from %s on port %d\n", inet_ntoa(their_addr.sin_addr),
                        htons(their_addr.sin_port));

		pid = fork();
		if (pid == 0) {
			/* In child process */
			close(sock);
			handle(newsock);
			return 0;
		}
		else {
			/* Parent process */
			if (pid == -1) {
				perror("fork");
				return 1;
			}
			else {
				close(newsock);
			}
		}
	}

    close(sock);

    return 0;
}

This server can only handle one client at a time, with up to 10 clients (the backlog argument to listen), being allowed to wait before connections are refused. This is suitable for a single-user desktop server or one in which connections are very short-lived.

#include <stdio.h>
#include <string.h> /* memset() */
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netdb.h>

#define PORT    "32001" /* Port to listen on */
#define BACKLOG 10  /* Passed to listen() */

void handle(int newsock)
{
	/* recv(), send(), close() */
}

int main(void)
{
    int sock;
	struct addrinfo hints, *res;
	int reuseaddr = 1; /* True */

	/* Get the address info */
	memset(&hints, 0, sizeof hints);
	hints.ai_family = AF_INET;
	hints.ai_socktype = SOCK_STREAM;
	if (getaddrinfo(NULL, PORT, &hints, &res) != 0) {
		perror("getaddrinfo");
		return 1;
	}

	/* Create the socket */
    sock = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
	if (sock == -1) {
		perror("socket");
		return 1;
	}

	/* Enable the socket to reuse the address */
	if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &reuseaddr, sizeof(int)) == -1) {
		perror("setsockopt");
		return 1;
	}

	/* Bind to the address */
    if (bind(sock, res->ai_addr, res->ai_addrlen) == -1) {
        perror("bind");
        return 1;
    }

	/* Listen */
    if (listen(sock, BACKLOG) == -1) {
        perror("listen");
        return 1;
    }

	freeaddrinfo(res);
 
	/* Main loop */
	while (1) {
		socklen_t size = sizeof(struct sockaddr_in);
		struct sockaddr_in their_addr;
		int newsock = accept(sock, (struct sockaddr*)&their_addr, &size);

		if (newsock == -1) {
			perror("accept");
		}
		else {
			printf("Got a connection from %s on port %d\n", 
					inet_ntoa(their_addr.sin_addr), htons(their_addr.sin_port));
			handle(newsock);
		}
	}

    close(sock);

    return 0;
}