C : "same file descriptors of all client connections" (client server programming) - c

Question

On every new client connection forked server process
different processes(other children of server i.e. exec) couldn't identify that client with same fd in used in forked child.
How can I differentiate client on other processes ??
accept call is returning same value if file descriptor new sockfd
/* server process */
#include <ctype.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <signal.h>
#define SIZE sizeof(struct sockaddr_in)
void catcher(int sig);
int newsockfd;
int main(void)
{
int sockfd;
char c;
struct sockaddr_in server = {AF_INET, 7000, INADDR_ANY};
static struct sigaction act;
act.sa_handler = catcher;
sigfillset(&(act.sa_mask));
sigaction(SIGPIPE, &act, NULL);
/* set up the transport end point */
if ( (sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
perror("socket call failed");
exit(1);
}
/* bind an address to the end point */
if ( bind(sockfd, (struct sockaddr *)&server, SIZE) == -1)
{
perror("bind call failed");
exit(1);
}
/* start listening for incoming connections */
if ( listen(sockfd, 5) == -1 )
{
perror("listen call failed");
exit(1) ;
}
for (;;)
{
/* accept a connection */
if ( (newsockfd = accept(sockfd, NULL, NULL)) == -1)
{
perror("accept call failed");
continue;
}
printf("New connection. File descriptor fd is %d\n",newsockfd);
/* spawn a child to deal with the connection */
if ( fork() == 0)
{
while (recv(newsockfd, &c, 1, 0) > 0)
{
c = toupper(c);
send(newsockfd, &c,1, 0);
}
/* when client is no longer sending information the socket can be closed and the child process terminated */
close(newsockfd);
exit (0);
}
/* parent doesn't need the newsockfd */
close(newsockfd);
}
}
void catcher(int sig)
{
close(newsockfd);
exit (0);
}
/* client process */
#include <ctype.h>
#include <sys/types.h>
#include<stdlib.h>
#include<stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define SIZE sizeof(struct sockaddr_in)
int main(void)
{
int sockfd;
char c, rc;
struct sockaddr_in server = {AF_INET, 7000};
/* convert and store the server's IP address */
server.sin_addr.s_addr = inet_addr("127.0.0.1");
/* set up the transport end point */
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
perror("socket call failed");
exit(1);
}
/* connect the socket to the server's address */
if ( connect (sockfd, (struct sockaddr *)&server, SIZE) == -1)
{
perror("connect call failed");
exit(1);
}
/* send and receive information with the server */
for(rc = '\n';;)
{
if (rc == '\n')
printf("Input a lower case character\n");
c = getchar();
send(sockfd, &c, 1, 0);
if (recv(sockfd, &rc, 1, 0) >0)
printf("%c", rc);
else
{
printf("server has died\n");
close(sockfd);
exit(1);
}
}
}

Answer 1

The file descriptor number is unique only within the process it exists in, and as soon as it's closed, it can be reused (for example the next time you call accept). It is not a good choice of "connection identifier".
In your case where you have a new process for each connection, the most natural identifier for a connection would be the process id - but you've thrown it away by not saving the return value of fork. In the parent process, fork returns the pid of the child process it created. You want to save this and use it later. In particular you can use it to kill the child process or identify when the child process exits (wait-family functions will tell you which child exited).
Of course if your whole model is to use a separate process for each connection, I'm not sure why you need to identify connections at all in the parent process. If each connection is not a completely independent process, you might do a lot better using threads instead of processes.