Re: question: spawning processes in a unix c++ application
- From: Tyler Littlefield <tyler@xxxxxxxxxxxxx>
- To: programmingblind@xxxxxxxxxxxxx
- Date: Fri, 18 Dec 2009 11:14:10 -0700
o, I got it! So, fork in the parent process returns 0, while it returns the pid
in the child process?
On Dec 18, 2009, at 10:48 AM, Jared Wright wrote:
> The switch statement is to keep your parent process from executing unwanted
> instructions after the fork is made. Although I thought process ID = 0 was
> the child process which is the reverse of what Laura's code indicates, but
> it's been a while since I've had to play with this sorta stuff. Either way,
> instructions after a fork call would be executed by both processes unless
> some control mechanism like this is used.
>
>
> Jared
>
> On 12/18/2009 11:33 AM, Tyler Littlefield wrote:
>> Hello laura,
>> thanks for the info, this really helps clarify things; I have a couple of
>> questions.
>> From what I understood, pipe creates an array of two dementions, 0 is input
>> and 1 is output or however you decide to use them. so, couldn't you just
>> send data to 0 and it would end up on the output end of the pipe?
>> Second, I don't really understand what your fork statement there does. I
>> know fork spawns another process, but I'm not quite sure what the whole case
>> 0: //parent process
>> bit was about. If you fork your going to be in the child process, no?
>>
>> Thanks,
>> On Dec 18, 2009, at 8:46 AM, qubit wrote:
>>
>>
>>> Hello Ty -- back on unix again?
>>> As long as you are on the same server you can fork an independent process to
>>> do what you want -- of course if you are looking at 2 systems with a network
>>> connection, you wouldn't use fork, but rather start two process manually,
>>> one on each side.
>>>
>>> The following took a little time to type... Hmm Hope it is useful to
>>> someone -- maybe I should put it in a tutorial on the website. I'll see if
>>> there is something about unix software development that hasn't been written
>>> yet and post it.
>>> *******
>>> I have in my possession a game I wrote for my nephew when he was small. It
>>> consists of a main board process and 5 players -- 4 computer generated
>>> monsters and one human player. All the monsters were spawned as independent
>>> processes, as was the player. The only difference between the player
>>> process and the monster processes was that the monsters communicated with
>>> the main board which would send it board state info, and the monster
>>> processes would then submit moves. The player process only communicated the
>>> users moves to the main board, which assumed the player didn't need the
>>> state information.
>>>
>>> Anyway, the seedy little reproductive details of the processes of the game
>>> were as follows (pseudo code):
>>>
>>> /* note: pipe returns a tiny array of 2 integer file identifiers.
>>> player_fd[0] is the front of the queue where moves are communicated -- it is
>>> input from the point of view of the main process, so it should be read.
>>> player_fd[1] is the rear of the queue where data is written.
>>> Now if you want to write from child's stdout to be read by the main
>>> process's input at the other end of the pipe, you have to somehow tie stdout
>>> in the child to the rear of the player_fd pipe. How do you do that? This is
>>> a twisted little trick. The secret is that stdin and stdout and stderr are
>>> in a global space that is inherited by the child process even after it calls
>>> exec. exec does not overwrite this data. This means that the parent and
>>> child processes can set up any environment it wants, including command line
>>> arguments to main, before calling exec to start the child.
>>> Noe for some pseudo code:*/
>>>
>>> // globals in file main.c
>>> //file descriptors associated with player and monsters -- note, monsters
>>> communicate 2 ways so need 2 descriptors
>>> static int player_fd = -1;
>>> static int monster_fd[4][2] = { { -1, -1}, { -1, -1 }, { -1, -1 },
>>> { -1, -1 } };
>>>
>>> int pid, mid[4]; // process ids of player and monsters
>>>
>>> // This function will fork and exec the player process
>>> spawn_player() {
>>> p = create a pipe -- I forget the syntax
>>> pid = fork()
>>> switch (pid) {
>>> case -1: fatal error
>>> case 0: // parent process
>>> // you are in the parent; close dangling end of pipe and return
>>> close(p[1]);
>>> player_fd = p[0];
>>> // note that rear of pipe is duplicated in the child process and the
>>> id will not be used by the parent process, so we close it.
>>> return
>>> default: // you are in a new process with process id == pid.
>>> // set up pipe the way you need it before exec
>>> // Note: stdin, stdout and stderr have file descriptors of 0, 1 and
>>> 2 in all new processes.
>>> // When you close stdout, you are freeing up number 1 to be used in
>>> the next file allocated -- in this case, the dup operation will set stdout
>>> to be the rear of the pipe.
>>> close(1) // stdout's process id
>>> dup(p[1]) -- this will allocate a new file 1 (stdout that will point
>>> to the rear of the pipe)
>>> close( p[0]) close(p[1]); // these are duplicates that hang onto
>>> resources.
>>> exec("player"...)
>>> }
>>> }
>>>
>>> I'll leave it as an exercise to write the spawn_monster function.
>>> Be sure to keep straight the nitty little differences between the ends of
>>> the pipe -- pipe[0] is always front, pipe[1] is always rear.
>>> Also keep process ids distinct from file ids.
>>> Hint: monsters need 2 pipes instead of one -- there is the move pipe where
>>> the monster writes info to the main process, and the info pipe which is used
>>> by the main process to send info to the monster.
>>> .
>>>
>>> Please reply with questions and I'll compile this with other things to post,
>>> if needed.
>>> Happy hacking.
>>> --le
>>>
>>> ----- Original Message -----
>>> From: "Tyler Littlefield"<tyler@xxxxxxxxxxxxx>
>>> To:<programmingblind@xxxxxxxxxxxxx>
>>> Sent: Thursday, December 17, 2009 11:52 PM
>>> Subject: question: spawning processes in a unix c++ application
>>>
>>>
>>> Hello all,
>>> I understand the basics of how fork works, but here's what I'm trying to do;
>>> maybe someone can give me a bit of a push in the right direction.
>>> I have a server that will be working with two interfaces, a telnet interface
>>> and a web interface. The telnet interface being part of the mud is already
>>> built in, but I would like to add in a web interface that would allow for
>>> viewing of generated data.
>>> What my idea was, was this:
>>> I want to somehow create a pipe that will allow for me to transport data
>>> back and forth between the server. I don't see why I should need to create
>>> two separate applications to serve one purpose, I would like to somehow
>>> spawn a new process from within the main server that will allow me to keep
>>> the pipe open and send/receive requests through it. Basically the server
>>> will send the data, and the web interface will generate the data, which will
>>> remove a lot of work off the mud its self.
>>> Can this forking easily be accomplished? I assume I could open the pipe and
>>> then fork from within a web initialization function toward the top of the
>>> code and then wait for a signal through the pipe to notify the web server
>>> that it can start the listening process, but I'm not quite sure how all this
>>> would work.
>>>
>>> Thanks,
>>>
>>> Tyler
>>>
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