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SHMAT(2) NetBSD System Calls Manual SHMAT(2)
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shmat, shmdt -- map/unmap shared memory
Standard C Library (libc, -lc)
shmat(int shmid, const void *shmaddr, int shmflg);
shmdt(const void *shmaddr);
shmat() 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
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 in
existence until it is removed by a call to shmctl(2) with the IPC_RMID
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.
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 call-
ing 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
shmdt() will fail if:
[EINVAL] shmaddr is not the start address of a mapped shared
ipcrm(1), ipcs(1), mmap(2), shmctl(2), shmget(2)
The shmat and shmdt system calls conform to X/Open System Interfaces and
Headers Issue 5 (``XSH5'').
Shared memory segments appeared in the first release of AT&T System V
NetBSD 8.0 June 17, 2002 NetBSD 8.0