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);
DESCRIPTIONshmat() 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 mem‐
ory 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 VALUESshmat() 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 call‐
ing 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 ALSOmmap(2), shmctl(2), shmget(2), compat(5)BSD August 17, 1995 BSD
