Multithreading server can't process multiple clients - c

Question

I am developing a simple server-client program where the client sends a number to the server and the server sends the number back.
I am trying to implement multithreading to it so multiple clients can send numbers.
The problem is that when a second client connects the 1st one freezes when about to receive the data back.
I have tried many things so far, but nothing worked.
I will post my code below if anyone can help me with it.
This is the server:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include<string.h> //strlen
#include<sys/socket.h>
#include<arpa/inet.h> //inet_addr
#include<unistd.h> //write
#include<fcntl.h> //close
void *thread_function(void *arg);
int socket_desc , client_sock , c , read_size;
struct sockaddr_in server , client;
char client_message[20000]="0";
int len=256;
int i=0;
char message[] = "Hello World";
int main() {
int res;
pthread_t a_thread;
void *thread_result;
//Create socket
socket_desc = socket(AF_INET , SOCK_STREAM , 0);
if (socket_desc == -1)
{
printf("Could not create socket");
}
puts("Socket created");
//Prepare the sockaddr_in structure
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = htons( 8888 );
//Bind
if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
{
//print the error message
perror("bind failed. Error");
return 1;
}
puts("bind done");
//Listen
listen(socket_desc , 3);
//Accept and incoming connection
puts("Waiting for incoming connections...");
for(;;){
c = sizeof(struct sockaddr_in);
//accept connection from an incoming client
client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
if (client_sock < 0)
{
perror("accept failed");
return 1;
}
puts("Connection accepted");
printf("Client connected with IP: %s\n", inet_ntoa(client.sin_addr));
pthread_create(&a_thread, NULL, thread_function, (void *)message);
pthread_detach(a_thread);
}
// return 0;
}
//-------------------------------------------------
//-------------------------------------------------
void *thread_function(void *arg) {
//Receive a message from client
while((read_size = recv(client_sock , client_message , 2000 ,0)) > 0 )
{
len=strlen(client_message);
//Send the message back to client
send(client_sock , client_message ,2000,0);
fflush(stdin);
fflush(stdout);
puts(client_message);
strncpy(client_message,"0",sizeof(client_message) - 1);
}
}
And this is the client:
#include<stdio.h> //printf
#include<string.h> //strlen
#include<sys/socket.h> //socket
#include<arpa/inet.h> //inet_addr
#include<unistd.h> //write
#include<fcntl.h> //close
int main(int argc , char *argv[])
{
int n;
char sn[200];
int sock;
struct sockaddr_in server;
char message[2000] , server_reply[2000];
int i=0;
char user[20];
char pass[20];
int sig;
//Create socket
sock = socket(AF_INET , SOCK_STREAM , 0);
if (sock == -1)
{
printf("Could not create socket");
}
puts("Socket created");
server.sin_addr.s_addr = inet_addr("127.0.0.1");
server.sin_family = AF_INET;
server.sin_port = htons( 8888 );
//Connect to remote server
if (connect(sock , (struct sockaddr *)&server , sizeof(server)) < 0)
{
perror("connect failed. Error");
return 1;
}
puts("Connected\n");
//keep communicating with server
n=1;
while(1)
{
printf("Enter message : ");
strncpy (message,"0",sizeof(message) - 1);
// itoa(n, sn, 10);
sprintf(sn, "%d", n);
strcpy(message,sn);
//gets(message);
//Send some data
send(sock , message , strlen(message) , 0);
recv(sock , server_reply , 2000 , 0);
puts("Server reply :");
puts(server_reply);
n++;
}
close(sock);
return 0;
}

Answer 1

You have only a single variable client_sock which is shared between all threads. This means each new accept will replace this variable with the new client socket so that essentially all your threads will work on the same (newest) socket and no thread will handle the previously established sockets.