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Full Discussion: Problems understanding pipes
Top Forums Programming Problems understanding pipes Post 302562703 by ab_tall on Friday 7th of October 2011 06:21:18 PM
Old 10-07-2011
Quote:
Originally Posted by Corona688
Nope. One pipe is one pipe, for a chain of 10 processes you need 9 pipes.

Not sure what you mean by "adjusting the FD". The FD's are just numbers, sure, but they're numbers representing things in the kernel's table of open files. Add three to it and the kernel won't give you pipe number three, it'll say "What? You never opened file #67."

6 is just a number, perhaps the 6th file your process happened to open, it doesn't mean "pipe number 6". Closing #6 doesn't close everyone else's #6.

It's completely okay to have the same file open and used in many different processes, too -- that's how shells work. When you run echo a or cat, they receive copies of the shell's own open file descriptors, and just write to those direct: The shell doesn't have to write it for them.

Pipes obviously know to wait until the process writing to them finishes before saying they're done, but that works for more than one process too. If you have two processes with copies of the write-end, the kernel will wait for both of them to die before the pipe gives up -- even if one of the open ends was just left open by accident.

Every process is independent. Close everything you don't need. fork() creates a new, independent process. Threads are something else entirely.

When you create a thread it works in the same process, literally sharing all the same memory, all the same files. Change it in one thread and it changes in all of them. That's why threads can be so tricky -- it's easy to rip the floor out from under your threads by altering something they're using simultaneously.
Regarding the last portion,

Quote:
When you create a thread it works in the same process, literally sharing all the same memory, all the same files. Change it in one thread and it changes in all of them. That's why threads can be so tricky -- it's easy to rip the floor out from under your threads by altering something they're using simultaneously.
What I meant to say is I am having a hard time debugging the child process once it execs, as gdb is attached to the parent.
(I thought as there is no explicit concept of threads in Linux, it would be OK to call the child process a thread, but i guess that lead to confusion.)

Quote:
the kernel will wait for both of them to die before the pipe gives up -- even if one of the open ends was just left open by accident.
This point is what I missed out. Will keep this in mind in the future.
 

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LEARN ABOUT V7

pipe

PIPE(2) 							System Calls Manual							   PIPE(2)

NAME

       pipe - create an interprocess channel

SYNOPSIS

       pipe(fildes)
       int fildes[2];

DESCRIPTION

       The pipe system call creates an I/O mechanism called a pipe.  The file descriptors returned can be used in read and write operations.  When
       the pipe is written using the descriptor fildes[1] up to 4096 bytes of data are buffered before the writing process is suspended.   A  read
       using  the  descriptor  fildes[0] will pick up the data.  Writes with a count of 4096 bytes or less are atomic; no other process can inter-
       sperse data.

       It is assumed that after the pipe has been set up, two (or more) cooperating processes (created by subsequent fork calls)  will	pass  data
       through the pipe with read and write calls.

       The Shell has a syntax to set up a linear array of processes connected by pipes.

       Read calls on an empty pipe (no buffered data) with only one end (all write file descriptors closed) returns an end-of-file.

SEE ALSO

       sh(1), read(2), write(2), fork(2)

DIAGNOSTICS

       The  function value zero is returned if the pipe was created; -1 if too many files are already open.  A signal is generated if a write on a
       pipe with only one end is attempted.

BUGS

       Should more than 4096 bytes be necessary in any pipe among a loop of processes, deadlock will occur.

ASSEMBLER

       (pipe = 42.)
       sys pipe
       (read file descriptor in r0)
       (write file descriptor in r1)

																	   PIPE(2)
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