Server not able to receive message from client - c

Question

I am using Ubuntu 12.05.I have been trying to implement Stop and wait protocol through C socket programming.I have created two programs,one featuring the server and other one the client. Expected working of the code is explained through comments
serversocket.c
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <error.h>
#include <string.h>
#include <sys/types.h>
#include <time.h>
int main(int argc,char *argv[])
{
int listenfd = 0,qlength=10,connfd = 0,t=5;
int n;
struct sockaddr_in serv;
struct sockaddr_in dest;
socklen_t socksize = sizeof(struct sockaddr_in);
char sendBuff[1024]="hi\n";
//time_t ticks;
listenfd = socket(AF_INET,SOCK_STREAM,0); //socket for listening to connection requests
memset(&serv,'0',sizeof(serv));
serv.sin_family = AF_INET;
serv.sin_addr.s_addr = htonl(INADDR_ANY);
serv.sin_port=htons(5000);
bind(listenfd,(struct sockaddr *)&serv,sizeof(serv)); //binding the socket to a port
listen(listenfd,2);
connfd=accept(listenfd,(struct sockaddr *)&dest,&socksize); //another socket for sending and receiving on the previously built socket
while(connfd){
printf("Incoming connection from %s-sending a hi...\n",inet_ntoa(dest.sin_addr));
send(connfd,sendBuff,strlen(sendBuff),0); //at first my server sends a hi
sleep(3);
n=recv(connfd,sendBuff,1024,0); //if hi received by the client,then server should receive "Message Received" from the client
sendBuff[n]='\0';
printf("%s",sendBuff);
connfd=accept(listenfd,(struct sockaddr *)&dest,&socksize); }
return 0;
}
clientsocket.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 sockfd = 0, n = 0,m=2;
char recvBuff[1024];
struct sockaddr_in serv_addr;
if(argc != 2)
{
printf("\n Usage: %s <ip of server> \n",argv[0]);
return 1;
}
memset(recvBuff, '0',sizeof(recvBuff));
if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
printf("\n Error : Could not create socket \n");
return 1;
}
memset(&serv_addr, '0', sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(5000);
if(inet_pton(AF_INET, argv[1], &serv_addr.sin_addr)<=0)
{
printf("\n inet_pton error occured\n");
return 1;
}
if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
{
printf("\n Error : Connect Failed \n");
return 1;
}
while(m--){ //m is 2 initially,I want that sending and receiving should be done 2 times
n = recv(sockfd,recvBuff,1024,0);
recvBuff[n]='\0'; //"hi" is received
printf("%s",recvBuff);
if(recvBuff=="hi\n")
send(sockfd,"Message Received",strlen("Message Received\n"),0);} //sending "Message received"
return 0;
}
Now sending messages from server to client works fine, but client to server(Message received) is creating problems.Neither it is giving an error, nor correct results,it just gives a blank screen.

Answer 1

You cannot compare strings using the "==" operator.
You need to use strcmp()
So....
if( strcmp( recvBuff, "hi\n" ) == 0 )
send(sockfd,"Message Received",strlen("Message Received\n"),0);
else
printf( "[%s] != [hi\n]\n", recvBuff );

Answer 2

Basically, you have three different issue in your code:
the string comparison (fixed by K Scott Piel).
The print of the received message are not coming out. You need to add '\n' in all your printf functions.
This is the most important issue: after the first communication between the server and the client, you are calling the accept function in the server code. From the man accept page, we can read
The accept() function shall extract the first connection on the
queue of pending connections, create a new socket with the same socket
type protocol and address family as the specified socket, and allocate
a new file descriptor for that socket.
So, after the first communication, your server is waiting for a new connection and your client is waiting for a message from the server. The second communication will never happen.
To solve this issue, you can use fork() API.
Here is a fix proposal using the fork():
clientsocket.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 sockfd = 0, n = 0,m=2;
char recvBuff[1024];
struct sockaddr_in serv_addr;
if(argc != 2)
{
printf("\n Usage: %s <ip of server> \n",argv[0]);
return 1;
}
memset(recvBuff, '0',sizeof(recvBuff));
if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
printf("\n Error : Could not create socket \n");
return 1;
}
memset(&serv_addr, '0', sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(5000);
if(inet_pton(AF_INET, argv[1], &serv_addr.sin_addr)<=0)
{
printf("\n inet_pton error occured\n");
return 1;
}
if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
{
printf("\n Error : Connect Failed \n");
return 1;
}
//m is 2 initially,I want that sending and receiving should be done 2 times
while(m--)
{
printf("Waiting from the server ...\n");
n = recv(sockfd,recvBuff,1024,0);
recvBuff[n]= '\0'; //"hi" is received
printf("%s\n", recvBuff);
if(!strncmp(recvBuff, "hi\n", strlen("hi\n")))
{
printf("Send ACK\n");
n = send(sockfd,"Message Received",strlen("Message Received\n"),0);
printf("%d Sent ... \n", n);
}
} //sending "Message received"
return 0;
}
serversocket.c
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <error.h>
#include <string.h>
#include <sys/types.h>
#include <time.h>
int main(int argc,char *argv[])
{
int listenfd = 0,qlength=10,connfd = 0,t=5;
int n;
struct sockaddr_in serv;
struct sockaddr_in dest;
socklen_t socksize = sizeof(struct sockaddr_in);
char sendBuff[1024]="hi\n";
pid_t pid;
int m = 2;
//time_t ticks;
listenfd = socket(AF_INET,SOCK_STREAM,0); //socket for listening to connection requests
memset(&serv,'0',sizeof(serv));
serv.sin_family = AF_INET;
serv.sin_addr.s_addr = htonl(INADDR_ANY);
serv.sin_port=htons(5000);
bind(listenfd,(struct sockaddr *)&serv,sizeof(serv)); //binding the socket to a port
printf("Server started ...\n");
listen(listenfd,2);
connfd=accept(listenfd,(struct sockaddr *)&dest,&socksize); //another socket for sending and receiving on the previously built socket
while(connfd > 0)
{
printf("Incoming connection from %s-sending a hi...\n",inet_ntoa(dest.sin_addr));
pid = fork();
if(pid) {
/* shall continue to listen to new client */
printf("Parent shall continue listening ... \n");
connfd=accept(listenfd,(struct sockaddr *)&dest,&socksize);
} else {
printf("Chlid process: Communication with the client ... \n");
while(m--)
{
printf("try %d - Sending %s ... \n", m, sendBuff);
send(connfd,sendBuff,strlen(sendBuff),0); //at first my server sends a hi
sleep(3);
n=recv(connfd, sendBuff, 1024, 0); //if hi received by the client,then server should receive "Message Received" from the client
sendBuff[n]='\0';
printf("Data received: [%s]\n", sendBuff); /* need to add '\n' so the print will be displayed */
strcpy(sendBuff, "hi\n");
}
printf("Child process will exit\n");
return 0;
}
}
return 0;
}