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Full Discussion: named pipe with persistent buffer
Top Forums UNIX for Advanced & Expert Users named pipe with persistent buffer Post 302102243 by heck on Tuesday 9th of January 2007 07:07:04 AM
Old 01-09-2007
named pipe with persistent buffer
Hey folks,
i need communicate between 2 processes in a reliable manner. The information delivery has to be guarenteed. I thought about proc 2 sending a signal to proc 1 when information has being written to disc and wirte() has been verified (sync/flush). The IPC method for the data is named pipes.

When proc1 simply writes the data to the named pipe, and the system crashes before proc2 could read it and persistently save it to disc the information is lost.

So my solution to this is:

proc1:
has opened a phys. file, tmpfile; and a named pipe
1. writes data to tmpfile on disc, verify write()
2. writes the data to pipe
3. waits for signal from proc 2

proc2:
has select or poll to the pipe
1. writes data to disc on pipe change, verifiy write()
2. sends signal to proc 1, meaning data is persistently saved.

prioc1:
4. emtpys tmpfile

what do you think about this idea?
Is the pipe necessary?
Could i as well read the physical tmpfile from proc2 instead of double write from proc1 to phys. file AND pipe. Performance?

If you have suggestions introducing completly different concepts, please consider that proc1 is an already written program and outputs the data to a physical file. I cant really change proc1, adding a sighandler and a sigsuspend along a tmpfile creation/ emptiyng and a double write are small changes, but already nearly to big for this process (testing). Regulatory reasons (risk).

My question is also, isn't there anything like a named pipe with persistent buffer on disc, and with fast buffer im mem for the usual work. the persistent buffer is copied to membuffer on open(). So data which was written to pipe but not read before crash is not lost (if the crash was after succesfull write to disc pipe internal). The write() call to this kind of pipe should return when data is assuredly written to disc, so the user program can assume once returned from the write() call, data is now delivered, guaranteed.

Sorry for the long story, but i hope you can help me along with this conceptual problem!

Thanks in advance!
Heck
 

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LEARN ABOUT MINIX

pipe

PIPE(2) 							System Calls Manual							   PIPE(2)

NAME

       pipe - create an interprocess communication channel

SYNOPSIS

       #include <unistd.h>

       int pipe(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 PIPE_MAX bytes of data are buffered before the writing process	is  suspended.	 A
       read using the descriptor fildes[0] will pick up the data.

       PIPE_MAX equals 7168 under Minix, but note that most systems use 4096.

       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.

       The signal SIGPIPE is generated if a write on a pipe with only one end is attempted.

RETURN VALUE

       The function value zero is returned if the pipe was created; -1 if an error occurred.

ERRORS

       The pipe call will fail if:

       [EMFILE]       Too many descriptors are active.

       [ENFILE]       The system file table is full.

       [ENOSPC]       The pipe file system (usually the root file system) has no free inodes.

       [EFAULT]       The fildes buffer is in an invalid area of the process's address space.

SEE ALSO

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

NOTES

       Writes may return ENOSPC errors if no pipe data can be buffered, because the pipe file system is full.

BUGS

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

4th Berkeley Distribution					  August 26, 1985							   PIPE(2)
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