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C fork Confusion :-?


 
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# 1  
Old 07-28-2008
C fork Confusion :-?

Hi,

I was trying to learn forking in C in UNIX. Somehow i still haven't been able to get the concept well. I mean, i do understand that fork creates an exact replica of the parent (other than the fact that parent gets the process id of the child and child gets 0 when fork is called). This is the point till which i am ok.
Now comes the confusion. If the child is an exact replica, the how does it contain the same file handles. For example lets take the case of a simple webserver. This is how the logic goes

1) the program calls socket() function
2) then it calls bind();
3) then it calls listen(on port 80);
4) waits of connection so it calls accept();
5) As soon as it gets a connection from the client, it forks a child and lets the child handle the request.

Since the child is the exact replica of the parent, the child will try to listen on port 80 too. But that port is already blocked by the parent process. So how does forking work in this case?
The parent and child cant be exact replicas as they both cant be listening on the same port and IP address at the same time.
So my question is: when a fork() function is called, will the child process run the program right from the beginning or will it run from after the fork function call

Thanks in advance
# 2  
Old 07-28-2008
Quote:
Originally Posted by ralpheno
Hi,

I was trying to learn forking in C in UNIX. Somehow i still haven't been able to get the concept well. I mean, i do understand that fork creates an exact replica of the parent (other than the fact that parent gets the process id of the child and child gets 0 when fork is called). This is the point till which i am ok.
Now comes the confusion. If the child is an exact replica, the how does it contain the same file handles. For example lets take the case of a simple webserver. This is how the logic goes

1) the program calls socket() function
2) then it calls bind();
3) then it calls listen(on port 80);
4) waits of connection so it calls accept();
5) As soon as it gets a connection from the client, it forks a child and lets the child handle the request.

Since the child is the exact replica of the parent, the child will try to listen on port 80 too. But that port is already blocked by the parent process. So how does forking work in this case?
The parent and child cant be exact replicas as they both cant be listening on the same port and IP address at the same time.
So my question is: when a fork() function is called, will the child process run the program right from the beginning or will it run from after the fork function call

Thanks in advance
The child process continues executing the program from the point that fork was called.

Regards
# 3  
Old 07-29-2008
More a socket programming question in a lot of ways.
Tcp based example, hth.
Fwiw you are right about the child having access to the listening descriptor (via dup()), but you won't be able to accept() on it in the child while in the event loop in the parent, so you could safely close it in the child; in fact that's safe programming.
Code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
#include <string.h>
#include <sys/wait.h>

#define BSZ 256
#define SRVPORT 81
#define MAXLISTEN 5

void handleconnect(int sck) {
int sz = 0;
char buf[BSZ];

                   if (sck < 0) {perror("socket descriptor invalid"); exit(1);}
                   while ((sz = recv(sck,buf,BSZ,0)) > 0) {
                         printf("\nRead %d bytes at epochtime %d = %s in child: %d\n\n",sz,time(NULL),buf,getpid());
                   }
                   printf("Client disconnected.. %d exiting.\n",getpid());
                   exit(0);
}

int rettcpsrvsock(int portno) {
int fd;
struct sockaddr_in srv;
                      
                       bzero(&srv,sizeof(srv));
                       if ( (fd = socket(AF_INET,SOCK_STREAM,0)) <= 0) {perror("socket()"); return -1;}
                 
                       srv.sin_addr.s_addr = INADDR_ANY;
                       srv.sin_family = AF_INET;
                       srv.sin_port = htons(portno);
                       bind(fd,(struct sockaddr *)&srv,sizeof(srv));
                       listen(fd,MAXLISTEN);
                       return fd;
}

int main(void) {
int sz, csck, lsck;
pid_t foop;
struct sockaddr_in cli;


                          if ( (lsck = rettcpsrvsock(SRVPORT)) < 0) {perror("rettcpsrvsock()"); return 1;}
                          sz =  sizeof(cli);
 
                          while (1) {
                                    if ( (csck = accept(lsck,&cli,&sz)) < 0) {perror("accept()"); return 1;}
                                    if (fork() == 0) {handleconnect(csck);}
                                   /*edit*/ close(csck);
                                    if ( (foop = waitpid(-1,NULL,WNOHANG)) > 0) {printf("Reaped child: %d.\n",foop);}
                          }
return 1; /*not reached*/
}


Last edited by ramen_noodle; 07-29-2008 at 11:07 PM.. Reason: Too quick, descriptor leak.

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