Shared memory Post: 9287

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Top Forums Programming Shared memory Post 9287 by Neo on Wednesday 24th of October 2001 08:44:05 PM

10-24-2001

Administrator

Only one process (at a time) may have an open file descriptor for a shared memory segment. The interprocess communication that is used to insure proper file access is the semaphore.

When a semaphore is used correctly (and it can be tricky!) the problem with shared memory segmentation error will be solved (more than likely, unless you have a pointer or other C programming error).

Here is a basic algorithm:
[list=1][*]Create a shared memory segment.[*]Create a semaphore.[*]Process A acquires the semaphore used for the shared memory segment.[*]Process A opens the shared memory.[*]Process A does cool things with shared memory.[*]Process A closed the shared memory segment.[*]Process A releases the semaphore.[*]Process B has been waiting for the semaphore to be released.[*]Now that semaphore has been released......[*]Process B acquires the semaphore used for the shared memory segment.[*]Process B opens the shared memory.[*]Process B does cool things with shared memory.[*]Process B closed the shared memory segment.[*]Process B releases the semaphore.[/list=1]

Yadda, yadda, yadda....... does this help?

Neo

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LEARN ABOUT REDHAT

ipcclean

IPCCLEAN(1)						  PostgreSQL Server Applications					       IPCCLEAN(1)

NAME

       ipcclean - remove shared memory and semaphores from an aborted PostgreSQL server

SYNOPSIS

       ipcclean

DESCRIPTION

       ipcclean removes all shared memory segments and semaphore sets owned by the current user. It is intended to be used for cleaning up after a
       crashed PostgreSQL server (postmaster(1)). Note that immediately restarting the server will also clean up shared memory and semaphores,	so
       this command is of little real utility.

       Only the database administrator should execute this program as it can cause bizarre behavior (i.e., crashes) if run during multiuser execu-
       tion. If this command is executed while a postmaster is running, the shared memory and semaphores  allocated  by  the  postmaster  will	be
       deleted. This will result in a general failure of the backend servers started by that postmaster.

NOTES

       This  script  is  a  hack,  but in the many years since it was written, no one has come up with an equally effective and portable solution.
       Since the postmaster can now clean up by itself, it is unlikely that ipcclean will be improved upon in the future.

       The script makes assumption about the format of output of the ipcs utility which may  not  be  true  across  different  operating  systems.
       Therefore, it may not work on your particular OS.

Application							    2002-11-22							       IPCCLEAN(1)