SHMAT(2) System Calls Manual SHMAT(2)

shmat, shmdtmap/unmap shared memory

#include <sys/shm.h>

void *
shmat(int shmid, const void *shmaddr, int shmflg);

shmdt(const void *shmaddr);

() 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. () 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.

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.

The shmat() system call will fail if:

The calling process has no permission to access this shared memory segment.
shmid is not a valid shared memory identifier. shmaddr specifies an illegal address.
The number of shared memory segments has reached the system-wide limit.
There is not enough available data space for the calling process to map the shared memory segment.

The shmdt() system call will fail if:

shmaddr is not the start address of a mapped shared memory segment.

#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.

mmap(2), shmctl(2), shmget(2), compat(5)

August 17, 1995 macOS 14.4