Writing to Socket - c

Question

I am making a simple 1 to 1 server-client chat application using sockets.Basically there is a server which can communicate with a single client at a time.What I am trying to do is client should send a string to server and the server has to send it back to the client with changing the case of the string(upper to lower and vice-versa).The problem is the string is sent to the server but the response from the server never came on account of that the client is unable to send other string.
Output of the program:-
root#User:~/Desktop/Aadil/SystemPracticum/Programs/Assignment5# ./Server 4000
the message from client is message1
root#User:~/Desktop/Aadil/SystemPracticum/Programs/Assignment5# ./Client localhost 4000
enter the message message1
enter the message message2
Thank You
Here is my code
Server.c
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <time.h>
void ChangeCase(char *string){
int i = 0;
while(string[i]){
// printf("converting\n");
if(string[i] <= 90 && string[i] >= 65)
string[i] += 32;
else
string[i] -= 32;
++i;
}
}
int main(int counter, char *string[]){
if(counter < 2){
perror("erro! please provide port no.\n");
}else{
int server_socket_file_descriptor,client_socket_file_descriptor,
port_no,message_length,client_length;
char buffer[256];//buffer to be used for storing messages
struct sockaddr_in server_address,client_address;
server_socket_file_descriptor = socket(AF_INET,SOCK_STREAM,0);
/*it creates new socket the first argument AF_INET is used for internet domain
and second argument SOCK_STREAM is used for stream socket
third argument 0 means the default protocol for stram socket which is tcp*/
if(server_socket_file_descriptor < 0)
perror("\t\t\t\t=====!!!cant create a socket!!!=====\n");
bzero((char*)&server_address,sizeof(server_address));//set all value to 0
//set port no. by converting port from char* to integer
port_no = atoi(string[1]);
/*now initialize the server_address
server_address is a struct of sockaddr_in type which has four field in it
we need to initialize 3 of them
*/
server_address.sin_family = AF_INET;
//convert port no. to network byte order
server_address.sin_port = htons(port_no);
//set server ip address to the machines ip address in my case it is 10.8.3.236
server_address.sin_addr.s_addr=INADDR_ANY;
/*now we need to bind the server with socket created*/
if(bind(server_socket_file_descriptor,(struct sockaddr*)&server_address,
sizeof(server_address)) < 0){
perror("\t\t\t\t\t====error in binding====\n");
return 0;
}
//since socket is bind correctly I am not checking for the error
listen(server_socket_file_descriptor,8);
/*listening to socket. 8 represent the maximum client that
can wait in queue to connect to the server*/
//we are done with the server :D
client_length = sizeof(client_address);
client_socket_file_descriptor = accept(server_socket_file_descriptor,
(struct sockaddr*)&client_address,
&client_length);
if(client_socket_file_descriptor < 0)
perror("\t\t\t\t unable to connect to client");
while(1){
bzero(buffer,256);
message_length = read(client_socket_file_descriptor,buffer,255);
if(message_length < 0)
perror("\t\t\t\t error in reading from socket\n");
printf("\t\t\t\tthe message from client is %s\n",buffer);
ChangeCase(buffer);
message_length = write(client_socket_file_descriptor,buffer,sizeof(buffer));
if(message_length < 0)
perror("\t\t\t\t error writing to socket\n");
}
return 0;
}
}
Client.c
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>
int main(int argc, char *argv[])
{
int file_descriptor,message_length,port_no;
char Buffer[256];//to store the message
//to store the address of the server to which we want to connect
struct sockaddr_in server_address;
struct hostent *server;//hostent defines the host computer on internet
if(argc < 3){
printf("\t\t\t please provide ip address and port no.\n");
return 1;
}
port_no = atoi(argv[2]);
if((file_descriptor = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
printf("\n Error : Could not create socket \n");
return 1;
}
memset(&server_address, '0', sizeof(server_address));
server_address.sin_family = AF_INET;
//convert port no. to network byte order
server_address.sin_port = htons(port_no);
//set server ip address to the machines ip address in my case it is 10.8.3.236
server_address.sin_addr.s_addr = INADDR_ANY;
if(connect(file_descriptor,(struct sockaddr*)&server_address,
sizeof(server_address))<0){
perror("error in connection\n");
return 1;
}
while(1){
memset(Buffer, '0',sizeof(Buffer));
printf("\t\t\t\t\tenter the message\n");
fgets(Buffer,255,stdin);
message_length = write(file_descriptor,Buffer,strlen(Buffer));
if(message_length<0)
perror("\t\t\t\terror in writing\n");
memset(Buffer,'0',sizeof(Buffer));
message_length = read(file_descriptor,Buffer,255);
if(message_length < 0)
perror("\t\t\terror in reading from buffer\n");
else{
printf("%s\n",Buffer);
}
}
return 0;
}

Answer 1

I suspect that the problem is located in this line in your server code:
message_length = write(client_socket_file_descriptor,buffer,sizeof(buffer));
Note that this line always sends 256 bytes back to the client. For a string like "message1", that means it will send back "MESSAGE1" followed by 248 NUL/zero bytes.
Depending on how the TCP stack decides to break up those bytes, your client's read() call may receive those bytes in different partial sequences, and if any partial sequence it receives starts with a NUL/zero byte, it will print out as an empty string.
In order to better see what's going on, you might replace this line in your client:
printf("%s\n",Buffer);
with something like this:
printf("[%s]\n",Buffer);
I'd also recommend changing your server to specify strlen(buffer) as the final argument to write() rather than sizeof(buffer).