This is a simple Remote Procedural Calling (RPC) Server. When Executed it listens for client and computes the factorial of the number requested by the client. For Client Code See rpc-client.c gist

rpc-server.c

#include <unistd.h> 
#include <stdio.h> 
#include <sys/socket.h> 
#include <stdlib.h> 
#include <sys/wait.h>
#include <sys/types.h>
#include <netinet/in.h> 
#include <string.h> 

#define PORT 12345 //Our Server will listen on this port

// This is a function that will be executed and the parameter will be recieved from client.
unsigned long int factorial(int a){
	if(a <= 1) return 1;
	else return factorial(a-1)*a;
}

// It converts client request into integer.
unsigned long int remote_function(char* buffer){
	int a = atoi(buffer);
	return factorial(a);
}

int main() {
    int server_fd, new_socket, valread;
    struct sockaddr_in address;
    int opt = 1;
    int addrlen = sizeof(address);
    char buffer[1024] = {0};
    // Creating socket file descriptor
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }
    printf("Socket Created\n");
    // Forcefully attaching socket to the port
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT,
                                                  &opt, sizeof(opt))){
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }
    printf("Socket Attached\n");
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY; //listen on all active interfaces
    address.sin_port = htons( PORT );
    // Forcefully attaching socket to the port 10000
    if (bind(server_fd, (struct sockaddr *)&address,
                                 sizeof(address))<0){
        perror("bind failed");
        exit(EXIT_FAILURE);
    }
    printf("Socket is listening on all interface on port %d\n",PORT);
    if (listen(server_fd, 3) < 0)
    {
        perror("listen");
        exit(EXIT_FAILURE);
    }
    if ((new_socket = accept(server_fd, (struct sockaddr *)&address,
                       (socklen_t*)&addrlen))<0)
    {
        perror("accept");
        exit(EXIT_FAILURE);
    }
   printf("Serving Client : %d\n",new_socket);
   const char* info = "SERVER :: You are connected to computer\nSERVER :: Send your number whose factorial is expected ? \n";
   send(new_socket, info, strlen(info), 0);
   valread = read(new_socket , buffer, 1024);
   printf("Client Requested factorial of : %s\n",buffer);
   printf("Computing factorial of : %s",buffer);
   //Executing the remote function.
   char result[21];
   sprintf(result, "%lu", remote_function(buffer));
   printf("\nReturning to client :  %s\n",result);
   send(new_socket, result, strlen(result), 0);	
   close(new_socket);
   printf("Closing connection from client\n");
   return 0;
}