multithreading concept in client server program - c

Question

my program only works as read operation for server and write operation for client side.How can I do read and write operations for both client and server using multithreading concept.
I'm able to write message on client side but not able to write on server side. please help me to solve this problem.
/*server function*/
int server(int argc,char *argv[])
{
int sock_fd,socknew_fd,portNo,cliLen;//socket file descriptor-sock_fd
char buff[256];
struct sockaddr_in serv_addr,cli_addr;
int n;
/*first call to socket function*/
sock_fd=socket(AF_INET,SOCK_STREAM,0); /*socket created of family type IP v4 ,socket type TCP and 0 indicates default protocol to be selected*/
if(sock_fd<0) //if -1(sock_fd) returned by socket() it shows socket cant be created
{
perror("error opening socket");
exit(0);
}
/*initialize socket structure*/
bzero((char *)&serv_addr,sizeof(serv_addr));//setting all default structure socket values to 0
portNo=atoi(argv[1]);
serv_addr.sin_family=AF_INET;
serv_addr.sin_addr.s_addr=INADDR_ANY;
serv_addr.sin_port=htons(portNo);
//initialize family,server address,portno which should be of network order
/*now bind the host address using bind() call*/
if(bind(sock_fd,(struct sockaddr *) &serv_addr,sizeof(serv_addr))<0)
{perror("error in binding");
exit(1);
}
/*start listening for client and be in sleep mode till any request arises from client*/
while(1)
{
listen(sock_fd,5);
cliLen=sizeof(cli_addr);
/*accept actual connection from client*/
socknew_fd=accept(sock_fd,(struct sockaddr *) &cli_addr,&cliLen);
if(socknew_fd<0)
{
perror("error in accept");
exit(1);
}
/*if connection is established start communicating*/
bzero(buff,256);
n=read(socknew_fd,buff,255);
if(buff[0]==32)
break;
if(n<0)
{perror("error in reading from socket");
exit(1);
}
printf("here is message:%s\n",buff);
/*write a response to client*/
n=write(socknew_fd,"I got your message",18);
if(n<0)
{perror("error writing in socket");
exit(1);
}
}
return 0;
}