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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FIFO(7) Linux Programmer's Manual FIFO(7)
NAME
fifo - first-in first-out special file, named pipe
DESCRIPTION
A FIFO special file (a named pipe) is similar to a pipe, except that it is accessed as part of the file system. It can be opened by multi-
ple processes for reading or writing. When processes are exchanging data via the FIFO, the kernel passes all data internally without writ-
ing it to the file system. Thus, the FIFO special file has no contents on the file system; the file system entry merely serves as a refer-
ence point so that processes can access the pipe using a name in the file system.
The kernel maintains exactly one pipe object for each FIFO special file that is opened by at least one process. The FIFO must be opened on
both ends (reading and writing) before data can be passed. Normally, opening the FIFO blocks until the other end is opened also.
A process can open a FIFO in nonblocking mode. In this case, opening for read only will succeed even if no-one has opened on the write
side yet, opening for write only will fail with ENXIO (no such device or address) unless the other end has already been opened.
Under Linux, opening a FIFO for read and write will succeed both in blocking and nonblocking mode. POSIX leaves this behavior undefined.
This can be used to open a FIFO for writing while there are no readers available. A process that uses both ends of the connection in order
to communicate with itself should be very careful to avoid deadlocks.
NOTES
When a process tries to write to a FIFO that is not opened for read on the other side, the process is sent a SIGPIPE signal.
FIFO special files can be created by mkfifo(3), and are indicated by ls -l with the file type 'p'.
SEE ALSO
mkfifo(1), open(2), pipe(2), sigaction(2), signal(2), socketpair(2), mkfifo(3), pipe(7)
COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-03 FIFO(7)