Re: question: spawning processes in a unix c++ application
- From: "qubit" <lauraeaves@xxxxxxxxx>
- To: <programmingblind@xxxxxxxxxxxxx>
- Date: Fri, 18 Dec 2009 18:01:11 -0600
Oh man...*writhe,writhe* -- you are right -- I did turn around the child and
parent cases in the switch statement. I can't believe after all that typing
that I made that big of a "thinko". *UGH*
Ok, tyler, to fix this thinko all you need to do is change "case 0" to
"default", and "default" to "case 0".
One switcheroo, taking care to leave the comments where they are.
I hope now it is more enlightening.
Jared, thanks for calling this to my attention. *blush*, *frowny*
More about the function in my next mail.
--le
----- Original Message -----
From: "Jared Wright" <wright.jaredm@xxxxxxxxx>
To: <programmingblind@xxxxxxxxxxxxx>
Sent: Friday, December 18, 2009 11:48 AM
Subject: Re: question: spawning processes in a unix c++ application
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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