Multithreaded client-server chat application in c language - c

Question

I wrote an application for communicating between two clients (one will run the server.c application, and the other one client.c).
Everything goes very good at this point, both sides (client and server) can send and receive messages (there are two processes in both sides: one for listening and printing messages, and one for receiving and sending back messages).
There is what I got so far:
client.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <signal.h>
#include "aes.h"
#define BSIZE 320
uint8_t key[] = "qwertyuioplkjhg";
uint8_t iv[] = "123456789098765";
void error(const char *msg) {
perror(msg);
exit(0);
}
int main(int argc, char *argv[]) {
int sockfd, portno, n, pid;
struct sockaddr_in serv_addr;
struct hostent *server;
char buffer[BSIZE];
char paddedData[BSIZE];
unsigned char crypted_data[BSIZE];
if (argc < 3) {
fprintf(stderr,"usage %s <hostname> <port>\n", argv[0]);
exit(0);
}
portno = atoi(argv[2]);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) {
error("ERROR opening socket");
}
server = gethostbyname(argv[1]);
if (server == NULL) {
fprintf(stderr, "ERROR, no such host\n");
exit(0);
}
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr, server->h_length);
serv_addr.sin_port = htons(portno);
if(connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) {
error("ERROR connecting");
}
//while(1) {
switch(pid = fork()) {
case -1:
error("ERROR fork");
case 0:
while(1) {
//printf("Please enter the message: ");
bzero(buffer, BSIZE);
//printf("Message: ");
fgets(buffer, BSIZE - 1, stdin);
strncpy(paddedData, buffer, BSIZE);
AES128_CBC_encrypt_buffer(crypted_data, (unsigned char*)paddedData, BSIZE, key, iv);
n = write(sockfd, crypted_data, BSIZE - 1);
if(n < 0) {
error("ERROR writing to socket");
}
}
default:
while(1) {
//bzero(buffer,256);
n = read(sockfd, buffer, BSIZE - 1);
AES128_CBC_decrypt_buffer((unsigned char*)paddedData, (unsigned char*)buffer, BSIZE, key, iv);
if(n < 0) {
error("ERROR reading from socket");
}
printf("<<server>>: %s", paddedData);
}
}
close(sockfd);
return 0;
}
and server.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include "aes.h"
#define BSIZE 320
uint8_t key[] = "qwertyuioplkjhg";
uint8_t iv[] = "123456789098765";
int numberOfConnections = 0;
void communications_handler(int);
void error(const char *msg) {
perror(msg);
exit(1);
}
int main(int argc, char *argv[]) {
int sockfd, newsockfd, portno, pid;
socklen_t clilen;
struct sockaddr_in serv_addr, cli_addr;
if(argc < 2) {
fprintf(stderr, "ERROR, no port provided\n");
exit(1);
}
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if(sockfd < 0) {
error("ERROR opening socket");
}
bzero((char*)&serv_addr, sizeof(serv_addr));
portno = atoi(argv[1]);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
if(bind(sockfd, (struct sockaddr*) &serv_addr, sizeof(serv_addr)) < 0) {
error("ERROR on binding");
}
listen(sockfd, 5);
clilen = sizeof(cli_addr);
while(1) {
/* [1] */
newsockfd = accept(sockfd, (struct sockaddr*)&cli_addr, &clilen);
numberOfConnections++;
printf("\nThere are %d clients connected!\n\n", numberOfConnections);
if (newsockfd < 0) {
error("ERROR on accept");
}
pid = fork();
if (pid < 0) {
error("ERROR on fork");
}
if (pid == 0) {
close(sockfd);
communications_handler(newsockfd);
exit(0);
}
else {
close(newsockfd);
}
}
close(sockfd);
return 0;
}
void communications_handler(int sock) {
int n, pid;
char buffer[BSIZE];
char paddedData[BSIZE];
unsigned char crypted_data[BSIZE];
switch(pid = fork()) {
case -1:
error("ERROR on fork");
case 0:
while(1) {
n = read(sock, buffer, BSIZE - 1);
AES128_CBC_decrypt_buffer((unsigned char*)paddedData, (unsigned char*)buffer, BSIZE, key, iv);
if(n < 0) {
error("ERROR reading from socket");
}
printf("<<client>>: %s", paddedData);
}
default:
while(1) {
bzero(buffer, BSIZE);
//printf("Message: ");
fgets(buffer, BSIZE - 1, stdin);
strncpy(paddedData, buffer, BSIZE);
AES128_CBC_encrypt_buffer(crypted_data, (unsigned char*)paddedData, BSIZE, key, iv);
for(int i = 0; i < numberOfConnections; i++) {
n = write(sock, crypted_data, BSIZE - 1);
}
if(n < 0) {
error("ERROR writing to socket");
}
}
}
}
Now I want to extend this program, by letting the server to accept multiple connections (I actually did this, in server.c, at [1]).
But there is now one problem: How can I implement the communication between two (or more) clients (the server will only accept new connections, read data from all connected clients, and send data back to all clients).
Can this be done with processes?

Answer 1

Have a look at Beej's guide to non-blocking socket programming: http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html#select