Having issues with chat program over TCP using C++ on Ubuntu 12.10

Question

I'm developing a chat program over TCP to allow me to communicate between two machines over WAN. I'm pretty new to C++ (coming from Java) and am very new to TCP, so don't go too hard on me! (: I've looked around on a fair amount of tutorials and continue to find only Echo programs, nothing that can leave a connection open for an extended amount of time to allow a chat like function. My current code looks like this:

#include "ClientManager.h"

ClientManager::ClientManager() {

}

void ClientManager::connectCom(char* ipAdd) {



portno = atoi(PORT);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) 
    error("ERROR opening socket");
server = gethostbyname(ipAdd);
if (server == NULL) {
    fprintf(stderr,"ERROR, no such host\n");
    exit(0);
}
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr, 
     (char *)&serv_addr.sin_addr.s_addr,
     server->h_length);
serv_addr.sin_port = htons(portno);
if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) 
    error("ERROR connecting");
}

void ClientManager::message(std::string msg) {
// printf("Please enter the message: ");

char * buffer = new char[msg.size() + 1];
std::copy(msg.begin(), msg.end(), buffer);
buffer[msg.size()] = '\0';
// bzero(buffer,256);
// fgets(buffer,255,stdin);
n = write(sockfd,buffer,strlen(buffer));
if (n < 0) 
     error("ERROR writing to socket");
bzero(buffer,256);
n = read(sockfd,buffer,255);
if (n < 0) 
     error("ERROR reading from socket");
printf("%s\n",buffer);
bzero(buffer,256);
}

void ClientManager::closeCom() {
close(sockfd);
}

void ClientManager::error(const char *msg)
{
perror(msg);
exit(0);
}

and my server manager looks like this:

#include "ServerManager.h"

ServerManager::ServerManager() {
    // int sockfd, portno, n;
 //         struct sockaddr_in serv_addr;
 //     struct hostent *server;

}

void ServerManager::openCom() {
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
     socklen_t clilen;


sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) 
    error("ERROR opening socket");
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = atoi(PORT);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
if (bind(sockfd, (struct sockaddr *) &serv_addr,
    sizeof(serv_addr)) < 0) 
    error("ERROR on binding");
listen(sockfd,5);

clilen = sizeof(cli_addr);
newsockfd = accept(sockfd, 
            (struct sockaddr *) &cli_addr, 
            &clilen);
if (newsockfd < 0) 
    error("ERROR on accept");
bzero(buffer,256);
// n = read(newsockfd,buffer,255);
// if (n < 0) error("ERROR reading from socket");
//  printf("Here is the message: %s\n",buffer);
// n = write(newsockfd,"I got your message",18);
// if (n < 0) error("ERROR writing to socket");
}

int ServerManager::readCom() {
bzero(buffer,256);
n = read(newsockfd,buffer,255);
if (n < 0) error("ERROR reading from socket");
    printf("Here is the message: %s\n",buffer);
n = write(newsockfd,"I got your message",18);
if (n < 0) error("ERROR writing to socket");
if (buffer[0] == '0') 
    return 1;
return 0;
}

void ServerManager::closeCom() {
close(newsockfd);
close(sockfd);

}

void ServerManager::error(const char *msg) {
perror(msg);
exit(1);
}

Both of these classes are implemented by separate main functions who call the functions necessary. I know that the code itself is ok for sending a single message- its been doing that for awhile now. Its just when client calls more than one message() that I experience errors, specifically, a segmentation fault. This only happens on the second message, the first one is sent and received appropriately.

If anyone could help me out, it would be greatly appreciated. Thanks!

Answer 1

In ClientManager::message you forget to delete the buffer at the end of the method. ALternatively, you should declare the buffer on the local stack as this:

char buffer[msg.size() + 1];

As such, the buffer will be automatically deallocated by the end of the call. And reading later code, you better do:

char buffer[256];

I think the code for reading the server's reply causes buffer overrun if your msg.size() < serverReply.size, even if you think you have 256 characters (you simply didn't allocated them at method start) . Please take care to this buffer length declaration and later handling, as C++ would let you write beyond it's end, corrupting neighbouring variables and leading to segfaults.