shmop(2)shmop(2)NAME
shmop: shmat, shmdt - shared memory operations
C SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
void *shmat(int shmid, void *shmaddr, int shmflg);
int shmdt (void *shmaddr);
DESCRIPTION
shmat attaches the shared memory segment associated with the shared
memory identifier specified by shmid to the data segment of the calling
process. The segment is attached at the address specified by one of the
following criteria:
If shmaddr is equal to (void *) 0, the segment is attached at the
first available address as selected by the system. If
(shmflg&SHM_SGI_ANYADDR) is true, the segment is attached at the
first available address, otherwise the system selects an available
address that is outside of the reserved area between 0x30000000 and
0x40000000. See mmap(2) and sgi_use_anyaddr(1) for more information
on the reserved range.
If shmaddr is not equal to (void *) 0 and (shmflg&SHM_RND) is true,
the segment is attached at the address given by (shmaddr - (shmaddr
modulus SHMLBA)).
If shmaddr is not equal to (void *) 0 and (shmflg&SHM_RND) is false,
the segment is attached at the address given by shmaddr.
The actual argument corresponding to shmaddr is evaluated by setting an
INTEGER*4 variable to the location of the process's data segment, in
other words, the value returned by %loc(3F).
Under certain conditions, performance can be greatly improved by sharing
mapping resources among processes attached to the same SHM segment. In
order to achieve sharing of mapping resources, the following conditions
must hold:
- the size of the SHM segment is a multiple of the constant SHMSUS (SHM
Segment Unit Size).
- the attaching address shmaddr is a multiple of SHMSUS.
- (shmflg&SHM_RDONLY) is false
The caller authorizes the system to use shared mapping resources by
setting the flag SHM_SHATTR in shmflg.
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If (shmflg&SHM_SHATTR) is true and the above conditions are met the
system uses shared mapping resources.
If (shmflg&SHM_SHATTR) is true and any of the above conditions are false
the system call fails.
Use of the SHM_SHATTR flag is discouraged in the following situations:
- if the calling process is likely to unmap (munmap(2)) or to re-map
(mmap(2)) large portions of the SHM segment, or
- if the calling process is likely to change the mapping attributes
(access permissions and cache policies) on large portions of the SHM
segment (mprotect(2)).
These behaviors cause the system to make private copies of mapping
resources and stop sharing, thereby defeating the purpose of the
SHM_SHATTR capability.
shmdt detaches from the calling process's data segment the shared memory
segment located at the address specified by shmaddr.
The segment is attached for reading if (shmflg&SHM_RDONLY) is true -
{READ}, otherwise it is attached for reading and writing {READ/WRITE}.
shmat fails and does not attach the shared memory segment if one or more
of the following are true:
EINVAL shmid is not a valid shared memory identifier.
EACCES Operation permission is denied to the calling process [see
intro(2)].
ENOMEM The available data space is not large enough to
accommodate the shared memory segment.
EINVAL shmaddr is not equal to zero, and the value of (shmaddr -
(shmaddr modulus SHMLBA)) is an illegal address.
EINVAL shmaddr is not equal to zero, (shmflg&SHM_RND) is false,
and the value of shmaddr is an illegal address.
EMFILE The number of shared memory segments attached to the
calling process would exceed the system-imposed limit.
EINVAL shmaddr is not a multiple of SHMSUS and
(shmflg&SHM_SHATTR) is true, or the size of the SHM
segment is not a multiple of SHMSUS and
(shmflg&SHM_SHATTR) is true.
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EACCES (shmflg&SHM_SHATTR) is true and (shmflg&SHM_RDONLY) is
true.
EINVAL shmdt fails and does not detach the shared memory segment
if shmaddr is not the data segment start address of a
shared memory segment.
SEE ALSOsgi_use_anyaddr(1), exec(2), exit(2), fork(2), intro(2), shmctl(2),
shmget(2), sproc(2).
DIAGNOSTICS
Upon successful completion, the return value is as follows:
shmat returns the data segment start address of the attached shared
memory segment.
shmdt returns a value of 0.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
NOTES
The user must explicitly remove shared memory segments after the last
reference to them has been removed.
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