client server programming buffer content not correct - c++

Question

I type two program one for client and one for server.
server is tcp concurrent echo server with select call,in order to use only one process to all client.
it uses apparent concurrency.
I develop program and run its working but after 3/4 message exchange bet client and server.
buffer content at server changes it showing current message with some character from the previous message.
I am not getting why this is happening.
Please anyone able to help me...
//Client Program
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <arpa/inet.h>
#include <iostream>
using namespace std;
#define MAXLINE 4096 /*max text line length*/
#define serv_PORT 3000 /*port*/
int main(int argc,char **argv)
{
int sockfd;
struct sockaddr_in servaddr;
char sendline[MAXLINE];
char recvline[MAXLINE];
/*int sendchars,recvchar;
char buf[MAXLINE];
*/
if (argc !=2)
{
cerr<<"Usage: Femto: <IP address of the serv"<<endl;
exit(1);
}
//Create a socket for the client
if ((sockfd = socket (AF_INET, SOCK_STREAM, 0)) <0)
{
cerr<<"Problem in creating the socket"<<endl;
exit(1);
}
//Creation of the socket
memset(&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr= inet_addr(argv[1]);
servaddr.sin_port = htons(serv_PORT);
//Connection of the client to the socket
if (connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr))<0)
{
cerr<<"Problem in connecting to the serv"<<endl;
exit(1);
}
while (fgets(sendline, MAXLINE, stdin) != NULL)
{
send(sockfd, sendline, strlen(sendline), 0);
if (recv(sockfd, recvline, MAXLINE,0) == 0)
{
cerr<<"The serv terminated"<<endl;
exit(1);
}
cout<< "String received from the serv: ";
fputs(recvline, stdout);
}
exit(0);
}
//Server program
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <iostream>
#include <sys/select.h>
#include <sys/time.h>
using namespace std;
#define MAXLINE 4096 /*max text line length*/
#define serv_PORT 3000 /*port*/
#define LISTENQ 65535
int main (int argc, char **argv)
{
int msock,ssock;
fd_set rfds;
fd_set afds;
int fd,nfds;
socklen_t client_len ;
char buf[MAXLINE];
struct sockaddr_in clientaddr, servaddr;
if ((msock = socket (AF_INET, SOCK_STREAM, 0)) <0)
{
cerr<<"Problem in creating the socket"<<endl;
exit(1);
}
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(serv_PORT);
bind (msock, (struct sockaddr *) &servaddr, sizeof(servaddr));
listen (msock, LISTENQ);
nfds=getdtablesize();
FD_ZERO(&afds);
FD_SET(msock,&afds);
while(1)
{
memcpy(&rfds,&afds,sizeof(rfds));
if(select(nfds,&rfds,(fd_set *)0,(fd_set *)0,(struct timeval * )0)<0)
{
cerr<<"Error in select";
// exit(1);
}
if(FD_ISSET(msock,&rfds))
{
//int ssock;
ssock= accept(msock,(struct sockaddr *)&clientaddr,&client_len);
if(ssock<0)
{
cerr<<"Accept error";
}
FD_SET(ssock,&afds);
}
int n;
for(fd=0;fd<nfds;++fd)
if(fd!=msock && FD_ISSET (fd,&rfds))
while ( (n = recv(fd, buf, MAXLINE,0)) > 0) {
cout<<"String received from and resent to the client:"<<endl;
puts(buf);
send(fd, buf, n, 0);
}
close(fd);
FD_CLR(fd,&afds);
}
}
output::
client-hi
server-hi
client-bye
server-bye
//after some message exchange
client-wru?
server-wru?
client- i m here
server-i am here u?

Answer 1

You're making the usual mistake of ignoring the count returned by recv(). The data in the buffer is only valid up to that count. The rest of it is unchanged from its previous value.
You're also ignoring the possibility of an error in bind(), listen(), send(), and recv().