Accepting sequential user input with socket program - c

Question

My question today is in regards to socket programming. I'm working on a program that creates a client and a server, where the client interacts with the records in the server database via given commands. I'm very new to socket programming, and i've been given some starter code to get the server and client connected and interacting. That being said, i'm very unfamiliar with which part of said code needs to be iterated in order to receive and send another message until the "stop" command is given. Until then, I want the user to continue to be able to send messages to the server.
(relevant) server code:
void error(char *msg)
{
perror(msg);
exit(1);
}
int main () {
int num = 1;
int del;
int sockfd, newsockfd, portno;
socklen_t clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
/*STEP 1*********************************************/
sockfd = socket(AF_INET, SOCK_STREAM, 0);
/*Make sure opening was successful*/
if (sockfd < 0)
error("ERROR opening socket");
/*STEP 2********************************************
0 out the server address*/
memset((char *) &serv_addr, 0, sizeof(serv_addr));
/*convert argument to int*/
portno = PORTNO;
/*this is always the same for this type of connection*/
serv_addr.sin_family = AF_INET;
/*INADDR_ANY is a macro that will find the current machine IP*/
serv_addr.sin_addr.s_addr = INADDR_ANY;
/*make sure byte order is correct
may be a no-op, but better safe than sorry*/
serv_addr.sin_port = htons(portno);
/*STEP 3********************************************/
if (bind(sockfd, (struct sockaddr *) &serv_addr,
sizeof(serv_addr)) < 0) {
error("ERROR on binding");
}
/*STEP 4********************************************
Open server up for listening, if you don't listen
you won't hear*/
listen(sockfd,5);
/*STEP 5******************************************/
clilen = sizeof(cli_addr);
/*Accept blocks until a connection with
a client is made. Returns a new socket
to communicate with the new connection.
Also receives address data about client*/
/*Communicate************************************/
char *array[20];
do {
newsockfd = accept(sockfd,
(struct sockaddr *) &cli_addr,
&clilen);
if (newsockfd < 0)
error("ERROR on accept");
memset(buffer, 0, 256);
printf("%s", "Enter Message:");
n = read(newsockfd,buffer,255);
printf("I read %d\n",n);
if (n < 0) error("ERROR reading from socket");
printf("Here is the message: %s",buffer);
n = write(newsockfd,"I got your message",18);
if (n < 0) error("ERROR writing to socket");
int i = 0;
char *p = strtok(buffer, " ");
while (p != NULL)
{
array[i++] = p;
p = strtok (NULL, " ");
}
if (strcmp(array[0], "put") == 0) {
insert(&front, array);
puts("PUT COMMAND");
} else if (strcmp(array[0], "get") == 0 && strcmp(array[1], "lname") == 0) {
puts("GET LNAME COMMAND");
} else if (strcmp(array[0], "get") == 0 && strcmp(array[1], "fname") == 0) {
puts("GET FNAME COMMAND");
} else if (strcmp(array[0], "get") == 0 && strcmp(array[1], "SID") == 0) {
puts("GET SID COMMAND");
} else if (strcmp(array[0], "get") == 0 && strcmp(array[1], "GPA") == 0) {
puts("GET GPA COMMAND");
} else if (strcmp(array[0], "delete") == 0 && strcmp(array[1], "SID") == 0) {
puts("DELETE COMMAND");
} else {
printf("%s\n", "Not a valid command.");
}
} while (strcmp(array[0], "stop") != 0);
return 0;
}
(relevant) client code:
#define PORTNO 20321
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <unistd.h>
void error(char *msg)
{
perror(msg);
exit(0);
}
int main(int argc, char *argv[])
{
/*set up ints for socket file descriptor
port number and return of read/write*/
int sockfd, portno, n;
int i;
/*structure for server info*/
struct sockaddr_in serv_addr;
/*used to hold the return of the function
that finds our server address, will
be copied into serv_addr*/
struct hostent *server;
/*for our message*/
char buffer[256];
/*make sure usage is correct*/
if (argc < 2) {
fprintf(stderr,"usage %s hostname\n", argv[0]);
exit(0);
}
/*convert our port number*/
portno = PORTNO;
/*create the socket*/
sockfd = socket(AF_INET, SOCK_STREAM, 0);
/*make sure it was made*/
if (sockfd < 0)
error("ERROR opening socket");
/*gethostbyname takes our host domain name and
resolves it to an address, there is a similar
function, gethostbyaddr that takes an address
and returns the same struct, struct hostent*/
server = gethostbyname(argv[1]);
/*make sure the host exists*/
if (server == NULL) {
fprintf(stderr,"ERROR, no such host\n");
exit(0);
}
/*0 out the server address stuct and set members*/
memset((char *) &serv_addr, 0, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
/*copy the data returned from gethostbyname
into the server address struct*/
memcpy((char *)server->h_addr_list,
(char *)&serv_addr.sin_addr.s_addr,
server->h_length);
serv_addr.sin_port = htons(portno);
/*Request a connection to the server through the socket we set up
make sure it connected, this function will also do the binding
of our socket and server info*/
if (connect(sockfd,(struct sockaddr*) &serv_addr,sizeof(serv_addr)) < 0) {
error("ERROR connecting");
}
for (i = 0; i < 3; i++) {
printf("Choose an operation: \n 1. put ln,fn,mi,SID,GPA\n 2. get lname\n 3. get fname\n 4. get SID\n 5. get GPA\n 6. delete (SID)\n 7. stop\nChoice: ");
/*Retrieve a message to send from the user*/
memset(buffer, 0, 256);
fgets(buffer,255,stdin);
/*send the message to the socket*/
printf("length is %d\n", (int)strlen(buffer));
n = write(sockfd,buffer,strlen(buffer));
if (n < 0)
error("ERROR writing to socket");
memset(buffer, 0, 256);
/*await an incoming message, read stops all process*/
n = read(sockfd,buffer,255);
if (n < 0)
error("ERROR reading from socket");
printf("%s\n",buffer);
return 0;
}
}
My server seems to be waiting for messages correctly, as I can restart my client program and send another message while the server is waiting and it is received as usual. But the way it stands, it will ask for another message, but it doesn't get properly sent/received like the first one did. For now, I have it set to loop 3 times just as a test. How do I properly create this read/write loop to the server?

Answer 1

Assuming you only want to have a single connection at a time then this, untested, version of you while loop should work.
while (true)
{
newsockfd = accept(sockfd,
(struct sockaddr *) &cli_addr,
&clilen);
if (newsockfd < 0)
error("ERROR on accept");
while (true)
{
memset(buffer, 0, 256);
printf("%s", "Enter Message:");
n = read(newsockfd,buffer,255);
printf("I read %d\n",n);
if (n < 0) error("ERROR reading from socket");
printf("Here is the message: %s",buffer);
n = write(newsockfd,"I got your message",18);
if (n < 0) error("ERROR writing to socket");
int i = 0;
char *p = strtok(buffer, " ");
while (p != NULL)
{
array[i++] = p;
p = strtok (NULL, " ");
}
if (strcmp(array[0], "put") == 0) {
insert(&front, array);
puts("PUT COMMAND");
} else if (strcmp(array[0], "get") == 0 && strcmp(array[1], "lname") == 0) {
puts("GET LNAME COMMAND");
} else if (strcmp(array[0], "get") == 0 && strcmp(array[1], "fname") == 0) {
puts("GET FNAME COMMAND");
} else if (strcmp(array[0], "get") == 0 && strcmp(array[1], "SID") == 0) {
puts("GET SID COMMAND");
} else if (strcmp(array[0], "get") == 0 && strcmp(array[1], "GPA") == 0) {
puts("GET GPA COMMAND");
} else if (strcmp(array[0], "delete") == 0 && strcmp(array[1], "SID") == 0) {
puts("DELETE COMMAND");
}
// Checking for stop command here
else if (strcmp(array[0], "stop") ==0)
{
close(newsockfd);
break;
}
else {
printf("%s\n", "Not a valid command.");
}
}
}