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January 25, 2018


Manual pages for a GNU/kFreeBSD system


shmat, shmdt - attach or detach shared memory


.Lb libc


.In sys/types.h
.In sys/ipc.h
.In sys/shm.h void * shmat int shmid const void *addr int flag int shmdt const void *addr


The shmat system call attaches the shared memory segment identified by shmid to the calling process’s address space. The address where the segment is attached is determined as follows:
  • If addr is 0, the segment is attached at an address selected by the kernel.
  • If addr is nonzero and SHM_RND is not specified in flag, the segment is attached the specified address.
  • If addr is specified and SHM_RND is specified, addr is rounded down to the nearest multiple of SHMLBA.
The shmdt system call detaches the shared memory segment at the address specified by addr from the calling process’s address space.


Upon success, shmat returns the address where the segment is attached; otherwise, -1 is returned and errno is set to indicate the error.

.Rv -std shmdt


The shmat system call will fail if:
No shared memory segment was found corresponding to shmid.
The addr argument was not an acceptable address.
Failed to attach the shared memory segment because the per-process kern.ipc.shmseg sysctl(3) limit was reached.
The shmdt system call will fail if:
The addr argument does not point to a shared memory segment.
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