Creating 3 process and piping a message

p00ndawg · #1 (**permalink**) 06-16-2009

sorry im very new to this but i am supposed to create 3 processes A,B, and C and have a direct link from a to b, b to c, and c to a.

here is my code. It does work, however if you look at what I bolded as long as my final read is p[0] it seems to always work, regardless of the bolded section.

can anyone explain this?

Code:

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>

#define MSGSIZE    14 
 
char *message = "hello, world!"; 
 
 
main() { 
char inbuf[MSGSIZE];
int p[3];

pid_t pid;
pid_t pid1;
 
if (pipe(p) == -1){ 
perror("pipe call"); 
exit(1); 
 
}


pid = fork();
pid1 = fork();


if(pid == -1){

perror("Fork failed");
exit(1);

}

if(pid | pid1 == 0)//process A
{
    close(p[0]); 
    write(p[1], message, MSGSIZE);
    read(p[1], message, MSGSIZE);
    write(p[2], message, MSGSIZE);
    
}

/*else if(pid1 == 0){

    close(p[1]);
    //read(p[1], message, MSGSIZE);
    write(p[2], message, MSGSIZE);
    
}*/


else{
    //parent process C
    close(p[2]); 
    read(p[0], inbuf, MSGSIZE);
    printf("Pipelined message return:%s\n", inbuf);
    wait(NULL); 
}

exit(0);
}

lagigliaivan · #2 (**permalink**) 06-22-2009

I don't understand very well this peace of code you wrote:

Code:

if(pid | pid1 == 0)//process A

This works as

Code:

 if ( pid | (pid1 == 0) )

and I don't understand really well what is the idea, could clarify it?

The code above is a OR operation and the if sentence will be performed according its result right?

---------- Post updated at 05:18 PM ---------- Previous update was at 05:16 PM ----------

I don't understand very well this piece of code you wrote:

if(pid | pid1 == 0)//process A

This works as

if ( pid | (pid1 == 0) )

and I don't understand really well it this is the idea, could you clarify it?

The code above is a OR operation and the if sentence will be performed according its result, right?

p00ndawg · #3 (**permalink**) 06-28-2009

Quote:

Originally Posted by lagigliaivan

I don't understand very well this peace of code you wrote:

Code:

if(pid | pid1 == 0)//process A

This works as

Code:

 if ( pid | (pid1 == 0) )

and I don't understand really well what is the idea, could clarify it?

The code above is a OR operation and the if sentence will be performed according its result right?

---------- Post updated at 05:18 PM ---------- Previous update was at 05:16 PM ----------

I don't understand very well this piece of code you wrote:

if(pid | pid1 == 0)//process A

This works as

if ( pid | (pid1 == 0) )

and I don't understand really well it this is the idea, could you clarify it?

The code above is a OR operation and the if sentence will be performed according its result, right?

yea I did that because i created two forks, but i wanted both to enter the switch statement.

tetsujin · #4 (**permalink**) 06-30-2009

Quote:

Originally Posted by p00ndawg

Code:

pid = fork();
pid1 = fork();

OK, first off, you've got a total of four processes here, not three... (Is that what you wanted? For the process I call A to spawn off three children? Or did you want three processes total?)

Process A (original process): pid = B, pid1 = C
Process B (first child of A): pid = 0, pid1 = D
Process C (second child of A): pid = B, pid1 = 0
Process D (child of B): pid = 0, pid1 = 0

Quote:

Code:

if(pid | pid1 == 0)//process A

This if statement will actually run for three of the four processes: A, C, and D:
Process A: (pid | pid1 == 0) = (B | C == 0) = (B | 0) = true
Process B: (pid | pid1 == 0) = (0 | D == 0) = (0 | 0) = false
Process C: (pid | pid1 == 0) = (B | 0 == 0) = (B | true) = true
Process D: (pid | pid1 == 0) = (0 | 0 == 0) = (0 | true) = true
(where true is some nonzero value...)

Quote:

Code:

{
    close(p[0]); 
    write(p[1], message, MSGSIZE);
    read(p[1], message, MSGSIZE);
    write(p[2], message, MSGSIZE);
}

p[2] is uninitialized. pipe() only creates two file descriptors: p[0], the read end of the pipe, and p[1], the write end of the pipe.

Also, from the problem description it sounded like you wanted each of the three communicating processes to have the ability to write messages to and receive messages from each of the other two communicating processes. This suggests you need to create at least three pipes (three invocations of pipe() in the parent process). I say "at least three pipes" because if these processes are running concurrently, allowing two processes to write to the same file descriptor could have unpredictable results. (Using write() to write a single, whole message will probably be safe - but if the message were large it's possible the write would block waiting for the reader to read some data out - in which case I think it's possible the other process's write() could wind up getting in before the first write() finishes its message. (Not sure, though. write() is a system call so the kernel may synchronize it...)

Remember that pipe() creates two file descriptors but just one pipe. A pipe with an input end and an output end. A file descriptor you get from pipe() is either read-only or write-only - you can never read and write from the same file descriptor, when the file descriptor was obtained with pipe().

p00ndawg · #5 (**permalink**) 06-30-2009

Quote:

Originally Posted by tetsujin

OK, first off, you've got a total of four processes here, not three... (Is that what you wanted? For the process I call A to spawn off three children? Or did you want three processes total?)

Process A (original process): pid = B, pid1 = C
Process B (first child of A): pid = 0, pid1 = D
Process C (second child of A): pid = B, pid1 = 0
Process D (child of B): pid = 0, pid1 = 0

This if statement will actually run for three of the four processes: A, C, and D:
Process A: (pid | pid1 == 0) = (B | C == 0) = (B | 0) = true
Process B: (pid | pid1 == 0) = (0 | D == 0) = (0 | 0) = false
Process C: (pid | pid1 == 0) = (B | 0 == 0) = (B | true) = true
Process D: (pid | pid1 == 0) = (0 | 0 == 0) = (0 | true) = true
(where true is some nonzero value...)

p[2] is uninitialized. pipe() only creates two file descriptors: p[0], the read end of the pipe, and p[1], the write end of the pipe.

Also, from the problem description it sounded like you wanted each of the three communicating processes to have the ability to write messages to and receive messages from each of the other two communicating processes. This suggests you need to create at least three pipes (three invocations of pipe() in the parent process). I say "at least three pipes" because if these processes are running concurrently, allowing two processes to write to the same file descriptor could have unpredictable results. (Using write() to write a single, whole message will probably be safe - but if the message were large it's possible the write would block waiting for the reader to read some data out - in which case I think it's possible the other process's write() could wind up getting in before the first write() finishes its message. (Not sure, though. write() is a system call so the kernel may synchronize it...)

Remember that pipe() creates two file descriptors but just one pipe. A pipe with an input end and an output end. A file descriptor you get from pipe() is either read-only or write-only - you can never read and write from the same file descriptor, when the file descriptor was obtained with pipe().

yes I actually wanted processes A, B, and C communicating with each other, but I only wanted to pipe a message from a->b->c. I didnt realize I had created 4 processes.
Is there anyway to just create 3 processes? how about a way to just get variable pipes communicating with each other?

Thanks for the awesome breakdown.

tetsujin · #6 (**permalink**) 06-30-2009

Quote:

Originally Posted by p00ndawg

yes I actually wanted processes A, B, and C communicating with each other, but I only wanted to pipe a message from a->b->c. I didnt realize I had created 4 processes.
Is there anyway to just create 3 processes? how about a way to just get variable pipes communicating with each other?

Sure. The reason you wound up with a total of four processes is because each call to fork() returns twice: once in the original process and once in the new one. So after you make the first call to fork(), there are two processes - both of which go on to call the second fork().

To create a total of three processes, make sure you only fork twice. You can do this by checking the return value of fork() to make sure you're in the process which should fork.

Code:

pid_b = fork();
if (pid_b == 0)
{
    /* fork() returned 0, so this is the new process, which I call process B. */
    pid_c = fork();
    if (pid_c == 0)
    {
        /* This is process C... */
    }
}

Now, if your communication channels only need to be such that A can send messages to B, and B can send messages to C, you wind up with something like this:

Code:

pipe(channel_ab);
 pid_b = fork();
if (pid_b > 0)
{
    /* Process A...  Send messages to B using channel_ab[1] */
    close(channel_ab[0]);
    write(channel_ab[1], msg, size);
} else if (pid_b == 0) {
    /* Process B */
    close(channel_ab[1]);

    pipe(channel_bc);
    pid_c = fork();
    if (pid_c > 0)
    {
        /* Still process B.  Receive messages from A using channel_ab[0], send messages to C using channel_bc[1]. */
        close(channel_bc[0]);
        msg_size = read(channel_ab[0], buffer, size);
        write(channel_bc[1], msg, size);
    } else if (pid_c == 0) {
        /* Process C.  Receive messages from B using channel_bc[0]. */
        msg_size = read(channel_bc[0], buffer, size);
    }
}

'Course there's various error checking and stuff you'd want to do in there that's not shown above - but that's the basic idea.

p00ndawg · #7 (**permalink**) 07-01-2009

Quote:

Originally Posted by tetsujin

Sure. The reason you wound up with a total of four processes is because each call to fork() returns twice: once in the original process and once in the new one. So after you make the first call to fork(), there are two processes - both of which go on to call the second fork().

To create a total of three processes, make sure you only fork twice. You can do this by checking the return value of fork() to make sure you're in the process which should fork.

Code:

pid_b = fork();
if (pid_b == 0)
{
    /* fork() returned 0, so this is the new process, which I call process B. */
    pid_c = fork();
    if (pid_c == 0)
    {
        /* This is process C... */
    }
}

Now, if your communication channels only need to be such that A can send messages to B, and B can send messages to C, you wind up with something like this:

Code:

pipe(channel_ab);
 pid_b = fork();
if (pid_b > 0)
{
    /* Process A...  Send messages to B using channel_ab[1] */
    close(channel_ab[0]);
    write(channel_ab[1], msg, size);
} else if (pid_b == 0) {
    /* Process B */
    close(channel_ab[1]);

    pipe(channel_bc);
    pid_c = fork();
    if (pid_c > 0)
    {
        /* Still process B.  Receive messages from A using channel_ab[0], send messages to C using channel_bc[1]. */
        close(channel_bc[0]);
        msg_size = read(channel_ab[0], buffer, size);
        write(channel_bc[1], msg, size);
    } else if (pid_c == 0) {
        /* Process C.  Receive messages from B using channel_bc[0]. */
        msg_size = read(channel_bc[0], buffer, size);
    }
}

'Course there's various error checking and stuff you'd want to do in there that's not shown above - but that's the basic idea.

oh ok thanks alot, I think i understand now . I have a project coming up where ill need to use some pipes and I this really fills in some of my blanks.

thanks again.

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