c - client server communication via socket in infinite loop - c

Question

I have the following code below for a client page and a server page, for which I want to send five messages from the client side to the server side (for this initial test, anyway). The code works fine if I don't put it in a loop, but then I can only send one message before both the client and server close. When I put the code in a loop, it seems to get stuck in some sort of deadlock. The results I get are below followed by the code. I've tried various changes and done extensive research but just can't make it work. Thanks in advance for any solutions offered.
Server side results
./server 5000
Here is the message: test
Client side results
./client localhost 5000
Please enter the message: test
I got your message
Please enter the message: another test
Please enter the message: test again
Please enter the message: test yet again
// The server stops acknowledging receipt of the message and I have to keep resetting both client and server
server.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main(int argc, char *argv[])
{
int sockfd, newsockfd, portno, clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
if (argc < 2) {
printf("Need at least two arguments\n");
exit(1);
}
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
printf("ERROR opening socket\n");
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = atoi(argv[1]);
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)
printf("ERROR on binding\n");
listen(sockfd,atoi(argv[1]));
clilen = sizeof(cli_addr);
while ((newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen)) >= 0) {
if (newsockfd < 0)
printf("ERROR on accept\n");
bzero(buffer,256);
n = read(newsockfd,buffer,255);
if (n < 0) printf("ERROR reading from socket\n");
printf("Here is the message: %s\n",buffer);
n = write(newsockfd,"I got your message\n",18);
if (n < 0) printf("ERROR writing to socket\ns");
}
close(sockfd);
return 0;
}
client.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
int main(int argc, char *argv[]) {
int sockfd, portno, n;
struct sockaddr_in serv_addr;
struct hostent *server;
char buffer[256];
if (argc < 3) {
printf("Not enough arguments\n");
exit(0);
}
portno = atoi(argv[2]);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
printf("Error opening socket\n");
server = gethostbyname(argv[1]);
if (server == NULL) {
printf("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)
printf("Error connecting\n");
int i = 0;
while (i < 5) {
printf("Please enter the message: ");
bzero(buffer, 256);
fgets(buffer, 255, stdin);
n = write(sockfd, buffer, strlen(buffer));
if (n < 0)
printf("Error writing to socket");
bzero(buffer, 256);
n = read(sockfd, buffer, 255);
if (n < 0)
printf("Error reading from socket");
printf("%s\n", buffer);
i++;
}
close(sockfd);
return 0;
}

Answer 1

You need to restructure your code to deal with the one connection you have before accepting another one. This is terrible code but is the basic template for what you want to do. Even so you can't deal with more than one connection at a time with this. You either have to multiplex or thread. Also, always read/write in a loop because you are not guaranteed to get a full request filled on any one call. Learn about and use perror or brothers.
void do_stuff(int sock)
{
for (int i = 0; i < 5; ++i)
{
bzero(buffer, 256);
n = read(newsockfd, buffer, 255);
if (n < 0)
printf("ERROR reading from socket\n");
printf("Here is the message: %s\n", buffer);
n = write(newsockfd, "I got your message\n", 18);
if (n < 0)
printf("ERROR writing to socket\ns");
}
}
while ((newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen)) >= 0)
{
if (newsockfd < 0)
printf("ERROR on accept\n");
do_stuff(newsockfd);
close(newsockfd);
}

Answer 2

I know its too late. But for someone who might have the same doubt I am trying to answer this.
Answer
How about changing the server program to this
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int sockfd, newsockfd, portno, clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
if (argc < 2) {
printf("Need at least two arguments\n");
exit(1);
}
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
printf("ERROR opening socket\n");
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = atoi(argv[1]);
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)
printf("ERROR on binding\n");
listen(sockfd,atoi(argv[1]));
clilen = sizeof(cli_addr);
while ((newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen)) >= 0) {
if (newsockfd < 0)
printf("ERROR on accept\n");
bzero(buffer,256);
while(true)
{
n = read(newsockfd,buffer,255);
if (n < 0)
{
printf("ERROR reading from socket\n");
break;
}
printf("Here is the message: %s\n",buffer);
n = write(newsockfd,"I got your message\n",18);
if (n < 0)
{
printf("ERROR writing to socket\ns");
break;
}
}
close(newsockfd);
}
close(sockfd);
return 0;
}
This is the changed part. I changed the code slightly such that the connection will close if any write or read error occurs.
while ((newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen)) >= 0) {
if (newsockfd < 0)
printf("ERROR on accept\n");
bzero(buffer,256);
while(true)
{
n = read(newsockfd,buffer,255);
if (n < 0)
{
printf("ERROR reading from socket\n");
break;
}
printf("Here is the message: %s\n",buffer);
n = write(newsockfd,"I got your message\n",18);
if (n < 0)
{
printf("ERROR writing to socket\ns");
break;
}
}
close(newsockfd);
}