AT&T UNIX™PC Unix System V Users Manual - tenox

INTRO ( 2) INTR0 (2)
A semaphore is a data structure that contains the following
members:
ushort
short
ushort
ushort
semval;
sempid;
semncnt;
semzcnt;
I* semaphore value *I
I* pid of last operation *I
I* # awaiting semval > cval *I
I* # awaiting semval = 0 *I
Semva.l is a non-negative integer. Sempid is equal to the process
ID of the last process that performed a semaphore operation on
this semaphore. Semncnt is a count of the number of processes
that are currently suspended awaiting this semaphore's semval to
become greater than its current value. Semzcnt is a count of the
number of processes that are currently suspended awaiting this
semaphore's semval to become zero.
Semaphore Operation Permissions.
In the semop(2) and semctl(2) system call descriptions, the permission
required for an operation is interpreted as follows:
00400 Read by user
00200 Alter by user
00060 Read, Alter by group
00006 Read, Alter by others
Read and Alter permissions on a semid are granted to a process if
one or more of the following are true:
The process's effective user ID is super-user.
The process's effective user ID matches sem_perm.[c]uid
in the data structure associated with semid and the
appropriate bit of the "user" portion (0600) of
sem_perm.mode is set.
The process's effective user ID does not match
sem_perm.[c]uid and the process's effective group ID
matches sem_perm.[c]gid and the appropriate bit of the
"group" portion (060) of sem_perm.mode is set.
The process's effective user ID does not match
sem_perm.[c]uid and the process's effective group ID
does not match sem_perm.[c]gid and the appropriate bit
of the "other" portion (06) of sem_perm.mode is set.
Otherwise, the corresponding permissions are denied.
Shared Memory Identifier
A shared memory identifier (shmid) is a unique positive integer
created by a shmget(2) system call. Each shmid has a segment of
memory (referred to as a shared memory segment) and a data
structure associated with it. The data structure is referred to as
shmid_ds and contains the following members:
struct
int
ushort
ushort
short
time_t
ipc_perm shm_perm;
shm_segsz;
shm_cpid;
shm_lpid;
shm_nattch;
shm_atime;
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I* operation permission struct *I
I* size of segment *I
I* creator pid *I
I* pid of last operation *I
I* number of current attaches *I
I* last attach time *I