SHMAT(2) OpenBSD 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);
DESCRIPTION
shmat() 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. Oth-
erwise, an attempt is made to map the shared memory segment at the ad-
dress 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
existent until it is removed by a call to shmctl(2) with the IPC_RMID
command.
RETURN VALUES
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 re-
turned, and the global variable errno is set to indicate the error.
ERRORS
shmat() 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 sys-
tem-wide limit.
shmdt() will fail if:
[EINVAL] shmaddr is not the start address of a mapped shared memory
segment.
SEE ALSO
ipcrm(1), ipcs(1), mmap(2), shmctl(2), shmget(2)
OpenBSD 3.4 August 17, 1995 1