shared memory and semaphores...

socket · #1 15-Feb-2004, 08:33

here's the deal...
i need two applications that i need to communicate through a shared memory segment...

lets say the user starts program A, i need it to create a shared memory segment and a semaphore,
when the user stats program B, i need it to attach itself to the segment and the semaphore.
i also need it to work the other way around, i cant tell which of the applications the users will start first.
i tried using a key for the semphore and the segment but i couldn't get it to work...

in both applications this is how i init the shared memory segment and the semaphore:

CPP / C++ / C Code:

	shmid = shmget(skey, SEGMENT_SIZE, IPC_CREAT | 0666);
	buffer = (char *) shmat(shmid, NULL, 0);

	semid = semget(skey,1,IPC_CREAT | 0666);
	semctl(semid, 0, SETALL, 0);

who do i set the properties that will create a segment if there isnt an segment exist with the key, if the segment exists i want to get the segment id so i could attach to it, same goes for the semaphore...
i cant figure it out from the man pages.......

thanks in advance for your help...

dsmith · #2 15-Feb-2004, 09:54

Hello socket. Quick note, gidforums has special codes for C syntax highlighting, if you use [c] and [/c], it will give you purdy highlighed code

I've never used shared app memory like this so I am grasping a little, but have you tried something like this: ***WARNING: UNCOMPILED CODE***

CPP / C++ / C Code:

if( (shmid = shmget(skey, SEGMENT_SIZE, IPC_CREAT | IPC_EXCL | 0666) ) == -1)
		if(errno == EEXSIT)
			shmid = shmget(skey, SEGMENT_SIZE, 0666);

This should check to see if the shared memory segment exists.

I haven't looked at the semaphore yet, but you can probably do something similar.

Sorry, I can't be of more help.

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