Open System Services System Calls Reference Manual (G06.27+, H06.04+)

shmget(2) OSS System Calls Reference Manual
DESCRIPTION
The shmget() function returns the shared memory identifier for the shared memory segment
identified by the key parameter. If the key parameter already has a shared memory identifier
associated with it and (shmflag & IPC_CREAT) is 0 (zero), that identifier is returned.
A new shared memory identifier, the associated shared memory table entry, and a new shared
memory segment of at least size bytes are created when either of the following is true:
• The value IPC_PRIVATE is used for the key parameter.
• The key parameter does not already have a shared memory identifier associated with it,
and (shmflag & IPC_CREAT) is not 0 (zero).
After creating a new shared memory identifier, the shmget() function initializes the shared
memory table entry associated with the identifier as follows:
• The creator’s user ID field (shm_perm.cuid) and owner’s user ID field (shm_perm.uid)
are set equal to the effective user ID of the calling process.
• The creator’s group ID field (shm_perm.cgid) and owner’s group ID field
(shm_perm.gid) are set equal to the effective group ID of the calling process.
• The least-significant nine bits of the access mode field (shm_perm.mode) are set equal
to the least-significant nine bits of the shmflag parameter.
• The shared memory segment size field (shm_segsz) is set to the value of the size parame-
ter.
• The following fields are all set to 0 (zero):
— shm_lpid, the process ID of the latest process that performed a shmat(),
shmdt(),orshmctl() operation
— shm_nattch, the number of processes that currently have this region attached
— shm_atime, the time of the last shmat() operation
— shm_dtime, the time of the last shmdt() operation
• The shm_ctime field is set equal to the current time. This field is updated when any of
the following events occur:
— The shared memory identifier is created.
— The permissions for the shared memory segment are changed.
— The shared memory identifier is removed.
• The process ID of the process that created the shared memory identifier (the shm_cpid
field) is set to the process ID of the calling process.
The shared memory identifier is used for the following purposes:
• It identifies a specific entry in the system-maintained shared memory table.
• It allows detection of references to a previously removed shared memory identifier.
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