password authentication in Client server model in C - c

Question

I am trying to authenticate client using a password and if the client enters different password he should be again asked to re-enter the password
Following is my server code:
while (1) {
printf("Expecting the pass\n");
ret = recv(sock, pass, sizeof(pass), 0);
if (ret == -1) {
perror("Error receiving message");
exit(1);
} else {
pass[strlen(pass) - 1] = '\0';
printf("Password received %s of length %d\n", pass, strlen(pass));
}
printf("Received\n");
if (strcmp("Abhishek", pass) == 0) {
printf("Password entered is correct\n");
snprintf(buffer, sizeof(buffer), "CORRECT");
break;
} else {
printf("Incorrect Password %s\n", pass);
snprintf(buffer, sizeof(buffer), "I");
}
printf("SENDING : %s\n",buffer);
ret = send(sock, buffer, strlen(buffer), 0);
if (ret == -1) {
perror("Error sending message");
exit(1);
}
}
And client side code is :
while (1) {
printf("Enter the password :");
fgets(buf, sizeof(buf), stdin);
buf[ret] = '\0';
ret = send(sockid, buf, strlen(buf), 0);
if (ret == -1) {
perror("Error sending message");
exit(1);
} else {
printf("Password sent\n");
}
memset(&buf[0], 0, sizeof(buf));
ret = recv(sockid, buf, sizeof(buf), 0);
printf("RECEIVED %s\n", buf);
if (ret == -1) {
perror("Error receiving message");
exit(1);
}
if (buf[0] != 'I') {
break;
}
}
When I execute them in different windows, I can see at server side:
Asking for password authentication
Expecting the pass
At client side:
Start the Admin Dialogue
Enter the password :
Now when I enter password in client window, it says:
Password sent
but Server is still at
Expecting the pass
Can somebody help me with that I am new to this concept

Answer 1

You did not provide a complete function: definitions for the arrays and various variables are missing. Are pass and buf indeed defined an arrays?
You are not sending the '\0' terminators. The message recv'd on the client side is not null terminated. strlen(pass) invokes undefined behavior. Unlikely to be the cause of your problem, but a significant bug nonetheless.
You need some kind of protocol to determine in the server and in the client when a complete message has been received. You should either use '\n' as an end of message indicator or possibly a '\0'.
Here is how you would force null termination on the server side:
printf("Expecting the pass\n");
ret = recv(sock, pass, sizeof(pass) - 1, 0);
if (ret == -1) {
perror("Error receiving message");
exit(1);
} else {
pass[ret] = '\0';
if (ret > 0 && pass[ret - 1] == '\n')
pass[--ret] = '\0';
printf("Password received %s of length %d\n", pass, ret);
}
But this is not sufficient to ensure that the full message has been received by the server, you really should recv until you get a linefeed, or some sort of protocol indicating a complete message.
On the client side, buf[ret] = '\0'; seems useless and potentially risky.