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fork()

This is a discussion on fork() within the C Programming forums, part of the General Programming Boards category; so i have a client server program, and everything works perfectly, but now i have to modify my server code ...

  1. #1
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    fork()

    so i have a client server program, and everything works perfectly, but now i have to modify my server code to allow for multiple clients, im not really sure if im using fork() correctly as i really dont understand what fork() does.

    ive tried googling for about an hour and still dont understand it. although my program still runs im not sure if im testing it correctly since it runs no differently than before i added fork(),(other than the fact that typing q in my client no longer makes the server program end).

    Code:
    #include <sys/types.h>
    
    #include <sys/socket.h>
    
    #include <netinet/in.h>
    
    #include <arpa/inet.h>
    
    #include <stdio.h>
    
    #include <stdlib.h>
    
    #include <errno.h>
    
    #include <string.h>
    
    #include <ctype.h>
    
    
    
    #define  PORT	3132		//port for server being used
    
    main(){
    
    	int rc;			//system cals return value
    
    	int s;			//socket descriptor
    	int connected;		//new connection's socket descriptor
    
    	int bytes_received;	//number of bytes received
    
    	int size_csa;		//size of clients address struct
    
    	int pid;
    
    	int i;			//iterator
    
    	char received[1024];	//array that stores received string
    
    	char changed[1024];	//array to store the changes of the received data
    
    	struct sockaddr_in sa;	//internet address struct for server
    
    	struct sockaddr_in ca;	//internet address struct for client
    
    	
    
            s = socket(AF_INET, SOCK_STREAM, 0);
    
    	//checks for an error with socket()
    
    	if (s < 0){
    
    		perror("Socket");
    
    		exit(1);
    
    	}
    
    
    
    	sa.sin_family = AF_INET;	//using internet address family
    
    	sa.sin_port = htons(PORT);	//copy port number in network byte order
    
    	sa.sin_addr.s_addr = INADDR_ANY;	//INADDR_ANY = Wild Card
    
    
    
    	//checks for an error with socket()
    
    	rc = bind(s, (struct sockaddr *)&sa, sizeof(sa));
    
    	if (rc){
    
    		perror("bind");
    
    		exit(1);
    
    	}
    
    	
    
    	//checks to see if there is an error with listen()
    
    	rc = listen(s,5);
    
    	if (rc){
    
    		perror("listen");
    
    		exit(1);
    
    	}
    
    	
    
            while(1){
    
    		//accept a connection using the address of the client
    
    		size_csa = sizeof(struct sockaddr_in);
    
    		connected = accept(s, (struct sockaddr *)&ca, &size_csa);
    		if (connected < 0){
    			perror("connect");
    			exit(1);
    		}
    
    		pid = fork();
    		if(pid < 0){
    			printf("fork error\n");
    			exit(1);
    		}
    		if(pid == 0){
    			while(1){
    
    				//clears the arrays received and changed
    
    				for(i = 0; i < 1024; i++){
    
    					received[i] = '\0';
    
    					changed[i] = '\0';
    
    				}
    
    				//adds a null terminator to the end of the received data
    
    				bytes_received = recv(connected,received,1024,0);
    
    				received[bytes_received] = '\0';
    
    	
    				//changes the data in the array received to store data with all caps
    
    				for(i = 0; i < strlen(received); i++)
    
    					received[i] = toupper(received[i]);
    
    	
    				//takes the data from the array received in reverse order and stores it into the array changed 
    
    				for(i = 0; i < strlen(received); i++)
    
    					changed[i] = received[strlen(received)-i-1];
    
    	
    				//if the string that was received was "q" the program ends, otherwise it continues
    
    				if (strcmp(changed , "Q\n") == 0){
    
    					close(connected);
    
    					break;
    
    				}
    
    			
    
    				else
    
    					send(connected, changed,strlen(changed), 0);  
    
    			}//end while loop
    		}//end if statement
    		else
    			continue;
    
    	}//end while loop
    
    	close(s);
    
    	return 0;
    
    }//end function main()
    1) Am i using fork() correctly? if not can someone explain it to me?
    2) when testing this in a linux terminal how can i test this program to make sure fork is working correctly?

    EDIT: if necessary i can post my client code
    Last edited by Jimb0; 04-29-2011 at 01:15 PM.
    Salem likes this.

  2. #2
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    Your Linux distro should have man pages for all the standard C library functions (section 3) as well as all the Linux specific C system call functions (section 2).
    Quote Originally Posted by man 2 fork
    DESCRIPTION
    fork() creates a new process by duplicating the calling process. The new process, referred to as the child, is an exact duplicate of the calling process, referred to as
    the parent, except for the following points:
    ...
    Basically, fork makes a copy of your current client. The new process is a child process of the process that called fork. When it comes into existence, it's in the same place in the code, as though it executed all the same instructions in the same order. All your global and local variables have the same values as right before the fork call (except the one holding the return value of fork, for obvious reasons). Almost everything is an exact copy of what the parent had.

    > 1) Am i using fork() correctly? if not can someone explain it to me?
    Not quite. Your usage of fork in general is fine, but given the fact that you're using sockets, you have some extra concerns. One of the concerns you is that fork gives the child a copy of all the open file descriptors, including network sockets. That means your recv on the parent and child will both receive from the same socket, which is bad. You need to look into using listen/accept. Your parent listens for connections, and when it gets a request, it forks off a child process to handle it. That child process accepts the connection and then communicates with the client.

    > 2) when testing this in a linux terminal how can i test this program to make sure fork is working correctly?
    If you don't get a "fork error" message, you can assume the fork call worked fine. If you really want to "see" it, check your 'ps -f' listing and you'll see the parent and child processes, with the PID and PPID columns showing the appropriate hierarchy.

    Here's a good network programming guide: Beej's Guide to Network Programming.

  3. #3
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    Quote Originally Posted by anduril462 View Post
    > 1) Am i using fork() correctly? if not can someone explain it to me?
    Not quite. Your usage of fork in general is fine, but given the fact that you're using sockets, you have some extra concerns. One of the concerns you is that fork gives the child a copy of all the open file descriptors, including network sockets. That means your recv on the parent and child will both receive from the same socket, which is bad. You need to look into using listen/accept. Your parent listens for connections, and when it gets a request, it forks off a child process to handle it. That child process accepts the connection and then communicates with the client.
    i think thats what im supposed to be doing though.

    my specifications for this project say that (probly should have posted this in my op post):
    -there should only be one socket opened per client run.
    -the server should be able to deal with multiple clients at once(fork will be necessary).

    or am i misunderstanding ur comment?

  4. #4
    and the hat of wrongness Salem's Avatar
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    I think to address anduril462's concern over open descriptors, you could begin your child process with
    close(s);

    Also, start to think about program structure as well. The whole inner while loop should be carved out into another function.
    Code:
    		pid = fork();
    		if(pid < 0){
    			printf("fork error\n");
    			exit(1);
    		}
    		if(pid == 0){
    			close(s);  // child doesn't want this
    			handleNewClient(connected);
    		} else {
    			close(connected);  // parent doesn't want this
    		}
    As a rough "rule of thumb", if you can't see the outermost open/close braces of a function on screen at the same time, then it is almost certainly too long.
    Another guide is to write down what the function does. Normally, it should be "This function does ....". If you find yourself writing "This function does ... and .... and ...", then that's another sign you should be at least thinking about a bit of reorganisation.

    Finally, a bug
    Code:
    	bytes_received = recv(connected,received,1024,0);
    	received[bytes_received] = '\0';
    1. The size parameter passed to recv() should be sizeof(received)-1
    The -1 is important, as it ensures there is always room to append a \0 to make it a string.
    2. ALWAYS check the return result before appending the \0 - it will be negative on error (and negative array subscripts are bad voodoo!)
    rags_to_riches likes this.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  5. #5
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    Quote Originally Posted by Salem View Post
    Code:
    		pid = fork();
    		if(pid < 0){
    			printf("fork error\n");
    			exit(1);
    		}
    		if(pid == 0){
    			close(s);  // child doesn't want this
    			handleNewClient(connected);
    		} else {
    			close(connected);  // parent doesn't want this
    		}
    ive seen numerous fork() examples exactly like this but it confuses me. in this code you call handleNewClient(connected) during the "child" part, which i assume is a function that does everything in my innermost while loop. if that is the case should i be calling handleNewClient(s) in the parent part? because the way it looks to me is that the parent doesnt get handled at all.

  6. #6
    and the hat of wrongness Salem's Avatar
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    Well the basic idea is that when accept() returns, you now have TWO connections that need processing. There are various ways of dealing with this, of which fork() is just one of them.

    So after a fork(), you have two processes, to handle two connections; so one process takes one connection, and the other process takes the other connection.

    By convention, the parent process keeps 's', because this is assumed to be a persistent socket, and the spawned child process takes 'connected' because this will close when the remote disconnects.

    > if that is the case should i be calling handleNewClient(s) in the parent part?
    No - your outer while loop is just responsible for calling accept() to detect new connections. When there is a new connection, you call fork() to create a separate process to handle just that connection (and nothing else).

    Compare your "while accept" loop as being the mail room guy in a company. He doesn't open letters, he just opens the bag and figures out where to send each letter to inside the company. The employees who get the letters are the forked processes who actually get to deal with it.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  7. #7
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    finally an explanation in layman's terms, i get it now.

    thank you
    Salem likes this.

  8. #8
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    As a rough "rule of thumb", if you can't see the outermost open/close braces of a function on screen at the same time, then it is almost certainly too long.
    Please tell the guys I work with (including my boss, the biggest offender)! They think nothing of 1000-line functions. It's a maintenance nightmare.

  9. #9
    and the hat of wrongness Salem's Avatar
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    > They think nothing of 1000-line functions.
    Show them this -> Cyclomatic complexity - Wikipedia, the free encyclopedia
    Then ask them how many paths exist in such large functions, and how well they think they're being tested.

    Or even just run gcov over such large functions with your real tests to see how inadequate it all is.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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