05-06-2010
I know how to use a shared memory segment in code. What I'm asking for is a command that will tell me all the processes PIDs that have called shmat for the memory. More specifically, all the processes that have called shmat and have not subsequently called shmdt. The kernel knows this, because the it holds the memory mapping for all processes. I just need to know how to make it tell me.
For more specifics, in the case I'm experiencing, some process has called shmctl asking for it to be deleted (second parameter passed as IPC_RMID) and until all processes have called shmdt the kernel must hold the segment. It flags this (as shown by ipcs) two ways: 1) it changes the key to 0xFFFFFFFF (in Linux it is 0x00000000), and 2) it shows a 'D' in the first character of the MODE column (don't see this in Linux). The first is not sufficient to say it's pending deletion because process private keys are also listed as 0xFFFFFFFF (0x00000000 in Linux). So, in Linux it seems there is no good way to see (via ipcs) that the segment is pending deletion. Either way, the shared memory segment will linger until every processes has called shmdt. Therefore, I need to know what processes need to be killed so it goes away.
P.S. It is quite annoying that the kernel switches the key.... I believe I know why it does it, and that's because a subsequent create for the key must succeed. But, it's just bloody annoying because at the very least it could hold the old value and list that with ipcs...but...whatever. Just means I'm guessing as to which key is being held on too longer than it should be.
Last edited by DreamWarrior; 05-06-2010 at 01:49 PM..
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SHMAT(2) BSD System Calls Manual SHMAT(2)
NAME
shmat, shmdt -- map/unmap shared memory
SYNOPSIS
#include <sys/shm.h>
void *
shmat(int shmid, const void *shmaddr, int shmflg);
int
shmdt(const void *shmaddr);
DESCRIPTION
shmat() maps the shared memory segment associated with the shared memory identifier shmid into the address space of the calling process. The
address at which the segment is mapped is determined by the shmaddr parameter. If it is equal to 0, the system will pick an address itself.
Otherwise, an attempt is made to map the shared memory segment at the address shmaddr specifies. If SHM_RND is set in shmflg, the system will
round the address down to a multiple of SHMLBA bytes (SHMLBA is defined in <sys/shm.h> ). A shared memory segment can be mapped read-only by
specifying the SHM_RDONLY flag in shmflg. shmdt() unmaps the shared memory segment that is currently mapped at shmaddr from the calling
process' address space. shmaddr must be a value returned by a prior shmat() call. A shared memory segment will remain existant until it is
removed by a call to shmctl(2) with the IPC_RMID command.
RETURN VALUES
shmat() returns the address at which the shared memory segment has been mapped into the calling process' address space when successful,
shmdt() returns 0 on successful completion. Otherwise, a value of -1 is returned, and the global variable errno is set to indicate the error.
ERRORS
The shmat() system call will fail if:
[EACCES] The calling process has no permission to access this shared memory segment.
[EINVAL] shmid is not a valid shared memory identifier. shmaddr specifies an illegal address.
[EMFILE] The number of shared memory segments has reached the system-wide limit.
[ENOMEM] There is not enough available data space for the calling process to map the shared memory segment.
The shmdt() system call will fail if:
[EINVAL] shmaddr is not the start address of a mapped shared memory segment.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
The include files <sys/types.h> and <sys/ipc.h> are necessary for both functions.
SEE ALSO
mmap(2), shmctl(2), shmget(2), compat(5)
BSD
August 17, 1995 BSD