Segmentation fault trying to send message to client from server - c

Question

I need to send a message to a client and then the client have to respond with an option. I get till the client and server connects, but both program end with "Segmentation Fault". Does anyone knows what this error means? Can someone give an idea to how to create a code that will make client and server interact. After receiving the option chosen by the client the server have to analyze it and send again a result to client.
My codes are:
Server
int
main(int argc, char **argv)
{
int listenfd, connfd;
socklen_t len;
struct sockaddr_in servaddr, cliaddr;
char buff[MAXLINE];
time_t ticks;
char message[MAXLINE]="This is the server";
char temp_scale[2];
char recvdata[MAXLINE + 1];
listenfd = Socket(AF_INET, SOCK_STREAM, 0);
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);/*----------------------------------------------------*/
servaddr.sin_port = htons(5555);
Bind(listenfd, (SA *) &servaddr, sizeof(servaddr));
Listen(listenfd, LISTENQ);
for ( ; ; )
{
len = sizeof(cliaddr);
connfd = Accept(listenfd, (SA *) &cliaddr, &len);
printf("Connection from %s, port %d\n",
Inet_ntop(AF_INET, &cliaddr.sin_addr, buff, sizeof(buff)),
ntohs(cliaddr.sin_port));
snprintf(message, sizeof(message), "%s\r\n");
Writen(connfd, message, strlen(message));
while ( (n = read(connfd, recvdata, MAXLINE)) > 0)
{
recvdata[n] = 0; /* null terminate*/
if (fputs(recvdata, stdout) == EOF)
err_sys("fputs error");
}
if (n < 0)
err_sys("read error");
Close(connfd);
}
}
Client
int
main(int argc, char **argv)
{
int sockfd, rd;
socklen_t len;
char recvline[MAXLINE + 1];
struct sockaddr_in servaddr, cliaddr;
char scale[2];
/*if (argc != 2)
err_quit("usage: a.out <IPaddress>");*/
if ( (sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
err_sys("socket error");
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(atoi(argv[2])); /*port passed through command line*/
if (inet_pton(AF_INET, argv[1], &servaddr.sin_addr) <= 0) /*The client translates the server address, passed on the command line*/
err_quit("inet_pton error for %s", argv[1]);
if (connect(sockfd, (SA *) &servaddr, sizeof(servaddr)) < 0)
err_sys("connect error");
len = sizeof(cliaddr);
Getsockname(sockfd, (SA *) &cliaddr, &len);
printf("Local Address is: %s\n",
Sock_ntop((SA *) &cliaddr, sizeof(cliaddr)));
printf("Iniciando read...\n");
while ( (rd = read(sockfd, recvline, MAXLINE)) > 0)
{
recvline[rd] = 0; /* null terminate*/
if (fputs(recvline, stdout) == EOF)
err_sys("fputs error");
}
if (rd < 0)
err_sys("read error");
printf("Enter option 'A' or 'B'");
send_scale(sockfd);
exit(0);
}
Thanks

Answer 1

Your server is probably faulting because of this:
snprintf(message, sizeof(message), "%s\r\n"); // <== no parameters
It is flat-out wrong. The snprintf() call has a format specifier that is expecting a char * to a null-terminated string, and you're passing it absolutely nothing. It is therefore grabbing a random value out of the stack, treating it as a pointer, and dereferencing it in attempt to fulfill the formatted request.
Without knowing the details of the API you're using (it clearly isn't standard BSD sockets just by the names alone) there isn't much more to go on.

Answer 2

Run your code in a debugger (for example gdb ./a.out) and find out in no time.

Answer 3

I don't know if it might help, but in C the null termination for strings is '\0', when you print your response:
recvdata[n] = 0; /* null terminate ----> this must be '\0'*/
if (fputs(recvdata, stdout) == EOF)
err_sys("fputs error");
you pad it whith a "0", so it will probably lead you to a segfault when fputs parse your string in order to print it.
Hope it helps!