Chat Program in C - c

Question

First off, this is homework, so please no outright answers. I am writing a back and forth chat program in C. I'm extremely new to C (just started learning for this class). Currently I have three files:
server.c
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdlib.h>
#include <strings.h>
#include <string.h>
#include "chat.h"
#define SERVER_PORT 1725
#define MAX_PENDING 5
#define MAX_LINE 256
int main()
{
struct sockaddr_in sin;
char buf[MAX_LINE];
int len;
int s, new_s;
struct chat_packet packet;
/* build address data structure */
bzero((char *)&sin, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(SERVER_PORT);
/* setup passive open */
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0)
{
perror("simplex-talk: socket");
exit(1);
}
if ((bind(s, (struct sockaddr *)&sin, sizeof(sin))) < 0)
{
perror("simplex-talk: bind");
exit(1);
}
listen(s, MAX_PENDING);
/* wait for connection, then receive and print text */
while(1)
{
if ((new_s = accept(s, (struct sockaddr *)&sin, &len)) < 0)
{
perror("simplex-talk: accept");
exit(1);
}
/* Stay in the following loop until CTRL+C */
while (len = recv(new_s, &packet, sizeof(packet), 0))
{
fputs(packet.sender_name, stdout);
fputs(": ", stdout);
fputs(packet.data, stdout);
fputs("\nYou: ", stdout);
while (fgets(buf, sizeof(buf), stdin))
{
if(strlen(buf) > 144)
{
printf("Your message is too long. Please enter a new message.\n");
continue;
}
else
{
buf[MAX_LINE-1] = '\0';
strncpy(packet.data,buf,144);
char sender[8] = "Mason"; /*should be argv[index of name]*/
strncpy(packet.sender_name, sender, 8);
send(new_s, &packet, sizeof(packet),0);
}
}
}
close(new_s);
}
}
client.c
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include "chat.h"
#define SERVER_PORT 1725
#define MAX_LINE 256
int main(int argc, char * argv[])
{
FILE *fp;
struct hostent *hp;
struct sockaddr_in sin;
char *host;
char buf[MAX_LINE];
int s;
int len;
struct chat_packet packet;
if (argc==2)
{
host = argv[1];
}
else
{
fprintf(stderr, "usage: simplex-talk host\n");
exit(1);
}
/* translate host name into peer's IP address */
hp = gethostbyname(host);
if (!hp) {
fprintf(stderr, "simplex-talk: unknown host: %s\n", host);
exit(1);
}
/* build address data structure */
bzero((char *)&sin, sizeof(sin));
sin.sin_family = AF_INET;
bcopy(hp->h_addr, (char *)&sin.sin_addr, hp->h_length);
sin.sin_port = htons(SERVER_PORT);
/* active open */
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0)
{
perror("simplex-talk: socket");
exit(1);
}
if (connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0)
{
perror("simplex-talk: connect");
close(s);
exit(1);
}
/* main loop: get and send lines of text */
while (fgets(buf, sizeof(buf), stdin))
{
if(strlen(buf) > 144)
{
printf("Your message is too long. Please enter a new message.\n");
continue; /*This allows the user to re-enter a message post-error*/
}
else
{
buf[MAX_LINE-1] = '\0';
strncpy(packet.data, buf, 144);
char sender[8] = "Abby"; /*should be argv[index of name]*/
strncpy(packet.sender_name, sender, 8);
send(s, &packet, sizeof(packet), 0);
recv(s, &packet, sizeof(packet),0);
fputs(packet.sender_name, stdout);
fputs(": ", stdout);
fputs(packet.data, stdout);
fputs("\nYou: ", stdout);
}
}
}
chat.h
#include <stdint.h> /* Needed for unsigned types */
#define MAX_DATA_LEN 144 /* So we are on 16-bit boundary */
#define USER_NAME_LEN 8
/* You must send this packet across the socket. Notice there are
* no pointers inside this packet. Why?*/
struct chat_packet {
u_short version; /* 16 bits -- Set to version 2 in code */
char sender_name[8]; /* 64 bits */
char data[MAX_DATA_LEN]; /* Message goes in here */
};
Everything except what is in the client and server while loops were given to me by my instructor. The base part of the assignment is getting back-and-forth chat functionality. I'm running everything in PuTTY using the command line. I duplicate the session and run client in one and server in the other. To run:
./client serverName
./server
I am able to go back and forth one time, and then nothing else sends or receives. I am still able to type, but the two sessions cannot see each other's messages past the first back and forth. I am not sure where my code is wrong. Any advice would be appreciated, as I'm very new to the language. Thanks in advance!

Answer 1

Okay, here's my hint: Think about what happens when you recv() zero characters. Also, check what happens when the server calls accept() vs. when the client calls connect().
You might also want to check the return values of your recv() calls more judiciously. (and send(), for that matter; if a call can fail, check its return value!) Here's a hint from the man recv page:
RETURN VALUES
These calls return the number of bytes received, or -1 if an error occurred.
Also, if you aren't familiar with a debugger (such as gdb), I would recommend learning it. In a pinch, you might consider adding printf() statements to your code, to figure out what is happening.
Also, think about where your "blocking calls" are. If you're not familiar with what it means to be a "blocking call", we call it "blocking" when you call a function, and that function doesn't return ("blocks") until some specified thing happens. For example, your accept() will block until a connection is accepted. Your fgets() will block until a line of text is received. send() would block if you've already sent too much data, and the buffer is full. recv() would block until you've received the specified number of bytes. recv() also has a behavior you might not expect, that you may need to account for:
If no messages are available at the socket, the receive call waits for a
message to arrive, unless the socket is nonblocking (see fcntl(2)) in
which case the value -1 is returned and the external variable errno set
to EAGAIN. The receive calls normally return any data available, up to
the requested amount, rather than waiting for receipt of the full amount
requested; this behavior is affected by the socket-level options
SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).
In your case, your packets might be small enough that you won't run into cases where you have to reassemble them yourself. But it couldn't hurt to check.
I think that gives you some avenues to explore...