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

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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 NETBSD

ipcrm

IPCRM(1)						    BSD General Commands Manual 						  IPCRM(1)

NAME

     ipcrm -- remove the specified message queues, semaphore sets, and shared memory segments

SYNOPSIS

     ipcrm [-M shmkey] [-m shmid] [-Q msgkey] [-q msqid] [-S semkey] [-s semid] ...

DESCRIPTION

     ipcrm removes the specified message queues, semaphores, and shared memory segments.  These System V IPC objects can be specified by their
     creation ID or any associated key.

     The following options are used to specify which IPC objects will be removed.  Any number and combination of these options can be used:

     -M shmkey
	     Mark the shared memory segment associated with key shmkey for removal.  This marked segment will be destroyed after the last detach.

     -m shmid
	     Mark the shared memory segment associated with ID shmid for removal.  This marked segment will be destroyed after the last detach.

     -Q msgkey
	     Remove the message queue associated with key msgkey from the system.

     -q msqid
	     Remove the message queue associated with the ID msqid from the system.

     -S semkey
	     Remove the semaphore set associated with key semkey from the system.

     -s semid
	     Removes the semaphore set associated with ID semid from the system.

     If the id or key argument is ``all'' then all entries of the appropriate type are removed.

     The identifiers and keys associated with these System V IPC objects can be determined by using ipcs(1).

SEE ALSO

     ipcs(1), shmat(2), shmctl(2), shmdt(2), shmget(2)

BSD
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