CODEWITHSUNDEEP
clinuxshared-memory
T.G.
T.G.

Reputation: 793

shmdt marked for deletion, c, linux

i'm trying to write some code in c dealing with sheared memory in linux, and I'm not sure when to use the system call shmdt, and when to use shmctl (segment_id, IPC_RMID, 0);

I looked at the man page of shmdt, and read this sentence: "shm_nattch is decremented by one. If it becomes 0 and the segment is marked for deletion, the segment is deleted."

what is that marking that the man page mentions? is it shmctl (segment_id, IPC_RMID, 0);? and dose that mean that shmctl (segment_id, IPC_RMID, 0); will not detach the segment if there is a process that is still connected?

if someone can explain what dose each call do, I'll be grateful. Thanks

Upvotes: 3

Views: 577

Answers (2)

anshkun
anshkun

Reputation: 115

to unmap the shared memory mapping from process address space use system call shmdt(shared memory start virtual address) but to delete the shared memory segment use shmctl() with IPC_RMID or ipcs command.
Shared memory segment data structures are maintained inside Linux kernel so deleting shared memory segment means removing or freeing the data structures from kernel.

Upvotes: 0

Ctx
Ctx

Reputation: 18410

shmdt() reverses the shmat()-operation.

shmat: Maps the shared memory segment in a processes address space

shmdt: Unmaps it again

shmctl (segment_id, IPC_RMID, 0); marks the segment for deletion, this means, it is the counteroperation to creating the shared memory segment with shmget(..., IPC_CREAT). If the reference counter is 0 when deleting, the segment is deleted immediately. Otherwise, deletion is deferred until the last process unmaps it (either explicitly with shmdt() or implicitly by terminating).

Upvotes: 4

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