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Full Discussion: Shared memory between two c program
Top Forums Programming Shared memory between two c program Post 302582304 by DreamWarrior on Thursday 15th of December 2011 12:54:05 PM
Old 12-15-2011
Yes, you either need a key or to pass the segment's ID. IPC_PRIVATE is meant to be used between two programs that can pass the ID, either because one forked the other and the ID is resident, or because they passed it in some other way. So, in IPC_PRIVATE world, you'd shmget in one and then pass the shmid to the other to shmat.

If you use the command "ipcs -m" you'll see all the shared memory segments. The KEY for IPC_PRIVATE (usually) is 0xffffffff, but you'll see a bunch of them. Each time you run this program you'll be creating another one. The ID for each will be different. Ultimately, it's the ID that determines which segment you attach to.

Think of shmget as a way to both create and lookup that ID. If you don't pass the same ID to each shmat call, you won't be looking at the same segment. Make sense?

Also, you probably mean sizeof(buffer) not sizeof(buffer[1]). The first will insure the segment can store the whole array, your way provides the segment only enough space to hold one element. Of course, this will cause an overflow.

edit: P.S. since you're never asking the shared memory segment be removed, it'll linger forever. You'll want to use ipcrm to get rid of it. Of course, you could also shmctl with the IPC_RMID command to ask it be removed. It'll hang around until all applications currently attached to it have detached. The kernel will detach the application when it is killed as well, so while it's good practice to shmdt, if you application dies in the middle the kernel will do it. Of course, on some O/Ses I've seen that fail and segments get "stuck" and the dumb machine needs a reboot to clean them up, lol.
Last edited by DreamWarrior; 12-15-2011 at 02:02 PM..
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LEARN ABOUT OPENDARWIN

shmget

SHMGET(2)						      BSD System Calls Manual							 SHMGET(2)

NAME

     shmget -- get shared memory area identifier

SYNOPSIS

     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>

     int
     shmget(key_t key, int size, int shmflg);

DESCRIPTION

     shmget() returns the shared memory identifier associated with the key key.

     A shared memory segment is created if either key is equal to IPC_PRIVATE, or key does not have a shared memory segment identifier associated
     with it, and the IPC_CREAT bit is set in shmflg.

     If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as
     follows:

     o	 shm_perm.cuid and shm_perm.uid are set to the effective uid of the calling process.

     o	 shm_perm.gid and shm_perm.cgid are set to the effective gid of the calling process.

     o	 shm_perm.mode is set to the lower 9 bits of shmflg.

     o	 shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0

     o	 shm_ctime is set to the current time.

     o	 shm_segsz is set to the value of size.

RETURN VALUES

     Upon successful completion a positive shared memory segment identifier is returned.  Otherwise, -1 is returned and the global variable errno
     is set to indicate the error.

ERRORS

     [EACESS]		A shared memory segment is already associated with key and the caller has no permission to access it.

     [EEXIST]		Both IPC_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key.

     [ENOSPC]		A new shared memory indentifier could not be created because the system limit for the number of shared memory identifiers
			has been reached.

     [ENOENT]		IPC_CREAT was not set in shmflg and no shared memory segment associated with key was found.

     [ENOMEM]		There is not enough memory left to created a shared memory segment of the requested size.

SEE ALSO

     shmctl(2), shmat(2), shmdt(2)

BSD
								  August 17, 1995							       BSD
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