![]() |
Hello and Welcome from United States to the UNIX and Linux Forums! Thank You for Visiting and Joining Our Global Community.
|
|
google unix.com
|
|||||||
| Forums | Register | Forum Rules | Links | Albums | FAQ | Members List | Calendar | Search | Today's Posts | Mark Forums Read |
| High Level Programming Post questions about C, C++, Java, SQL, and other programming languages here. |
More UNIX and Linux Forum Topics You Might Find Helpful
|
||||
| Thread | Thread Starter | Forum | Replies | Last Post |
| URGENT:::Can anybody help me in creating message queue appliction?? | arunchaudhary19 | High Level Programming | 9 | 11-20-2007 07:15 AM |
| creating 10 process | kpkant123 | Shell Programming and Scripting | 2 | 05-21-2007 05:28 PM |
| creating child process | Confuse | High Level Programming | 12 | 05-27-2005 11:48 AM |
| Redirection or piping error message | mariner | Shell Programming and Scripting | 2 | 05-10-2005 03:04 PM |
| How to know a new file is in process of creating? It has not been closed. | linkjack | High Level Programming | 2 | 02-11-2003 01:55 PM |
![]() |
|
|
LinkBack | Thread Tools | Search this Thread | Rate Thread | Display Modes |
|
||||
|
Creating 3 process and piping a message
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);
}
Last edited by vino; 06-17-2009 at 12:52 AM.. |
|
||||
|
Hi
I don't understand very well this peace of code you wrote:
Code:
if(pid | pid1 == 0)//process A Code:
if ( pid | (pid1 == 0) ) 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? |
|
||||
|
Quote:
|
|
||||
|
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:
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:
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(). |
|
||||
|
Quote:
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. Last edited by p00ndawg; 06-30-2009 at 09:29 PM.. |
|
||||
|
Quote:
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... */
}
}
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);
}
}
|
|
||||
|
Quote:
thanks again. |
| Sponsored Links | ||
|
|
![]() |
| Bookmarks |
| Thread Tools | Search this Thread |
| Display Modes | Rate This Thread |
|
|