[Challenge] Write a full web application in C - part 1, make the server wake up

#c #linux

Hello everyone, today I have started a challenge for myself. I must build a web application in the C programming language. In this challenge, I will build an online book store website and I use C for backend service. I hope you interested in this challenge.

This post is just part 1 of this challenge. In this post, I will write a simple TCP/IP socket server on the Unix-liked system (Linux, macOS...). Okay, let's go.

Step 1: Create soc.h file

Write socket creation function
In this function, I create a socket, bind socket to a port number and make it listen.

int open_tcp_soc(int listen_port) {
    int sd;
    sd = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in server_addr;
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = htons(INADDR_ANY);
    server_addr.sin_port = htons(listen_port);
    if ((bind(sd, (struct sockaddr*)&server_addr,sizeof(server_addr))) < 0)
        return -1;
    if (listen(sd, 10) != 0)
        return -1;
    return sd;
}

Write a function read data from socket

int receive_message(int socket, char* buf, int len)
{
    char *s = buf;
    int slen = len;
    int c = recv(socket, s, slen, 0);
    if (c < 0)
        return c;
    else if (c == 0)
        buf[0] = '\0';
    else
        s[c]='\0';
    return len - slen;
}

Write a function send data to socket

int send_message(int socket, const char* msg)
{
    int result = send(socket, msg, strlen(msg), 0);
    return result;
}

I put three above functions into file soc.h. This file include some others header files:

#ifndef __SOC_H__
#define __SOC_H__
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <unistd.h>

// functions here

#endif

Step 2: Build main.c file

Write signal handler function
This function handles a SIGTERM, SIGINT signal send into the main application. This makes my program exits when I press Ctrl+C.

void signal_handler(int sig)
{
    printf("\nserver exited\n");
    close(server);
    exit(0);
}

I register this handler with function:

signal(SIGINT,signal_handler);

Write a request handler function
This function response a simple message: "Hello World" when client request to my server.

char* handle_request(const char* msg) {
    char* response_text = (char*)malloc(REQUEST_SIZE);
    sprintf(response_text, "HTTP/1.1 200 OK\n\nHello World\n");
    return response_text;
}

Make server listen
Juts call function I created in step 1

#define LISTEN_PORT 8080

int server;
printf("server is listening on port %d\n", LISTEN_PORT);
server = open_tcp_soc(LISTEN_PORT);

Receive client request and send a response to client

struct sockaddr_storage client_addr;
unsigned int address_size = sizeof(client_addr);
int connect;
char* message = (char*)malloc(REQUEST_SIZE);
connect = accept(server,(struct sockaddr *)&client_addr,&address_size);
struct sockaddr_in *coming = (struct sockaddr_in *)&client_addr;
unsigned char *ip = (unsigned char *)&coming->sin_addr.s_addr;
unsigned short port = coming->sin_port;
if (connect < 0)
{   
    // can't accept connect from client
    printf("can not connect with %d.%d.%d.%d:%d\n", ip[0], ip[1], ip[2], ip[3], port);
    continue;
}
printf("connected with %d.%d.%d.%d:%d\n", ip[0], ip[1], ip[2], ip[3], port);

receive_message(connect, message, REQUEST_SIZE);
send_message(connect, handle_request(message));

Make server run forever
I put my server handler step into a forever loop. This because my server needs to listen to requests and send responses over and over again.

while(1)
{
    connect = accept(server,(struct sockaddr *)&client_addr,&address_size);
    struct sockaddr_in *coming = (struct sockaddr_in *)&client_addr;
    unsigned char *ip = (unsigned char *)&coming->sin_addr.s_addr;
    unsigned short port = coming->sin_port;
    if (connect < 0)
    {   
        // can't accept connect from client
        printf("can not connect with %d.%d.%d.%d:%d\n", ip[0], ip[1], ip[2], ip[3], port);
        continue;
    }
    printf("connected with %d.%d.%d.%d:%d\n", ip[0], ip[1], ip[2], ip[3], port);

    receive_message(connect, message, REQUEST_SIZE);
    send_message(connect, handle_request(message));
}

Make my server can handle multiple requests at the same time
You can see that my server can handle only one request at the same time. If my handle_request function takes too long to execute, other requests will be waiting for the current request. So, I can handle this by using the fork system call. You can read more about the fork system call here: http://man7.org/linux/man-pages/man2/fork.2.html
After my server connected with the client, my server will fork a child process to handler request and sen response to the client.

pid_first = fork();
if (pid_first == 0)
{
    // clear signal handler in the child process
    signal(SIGINT, SIG_DFL);
    pid_t pid_first;
    pid_t pid_second;
    pid_second = fork();
    if (pid_second == 0)
    {
        signal(SIGINT, SIG_DFL);
        receive_message(connect, message, REQUEST_SIZE);
        send_message(connect, handle_request(message));
        close(connect);
    }
    close(connect);
    exit(0);
}
wait(NULL);
close(connect);