SHMAT(2) BSD Programmer's Manual SHMAT(2)NAME
shmat, shmdt - map/unmap shared memory
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#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 ad-
dress at which the segment is mapped is determined by the shmaddr parame-
ter. If it is equal to 0, the system will pick an address itself. Other-
wise, 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 ex-
istent until it is removed by a call to shmctl(2) with the IPC_RMID com-
mand.
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.
ERRORSshmat() will fail if:
[EACCES] The calling process has no permission to access this shared
memory segment.
[ENOMEM] There is not enough available data space for the calling
process to map the 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.
shmdt() will fail if:
[EINVAL] shmaddr is not the start address of a mapped shared memory
segment.
SEE ALSOipcrm(1), ipcs(1), mmap(2), shmctl(2), shmget(2)MirOS BSD #10-current August 17, 1995 1
