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<H4><A NAME="SECTION00743210000000000000">Kernel <TT>semid_ds</TT> structure</A></H4>
<P>
As with message queues, the kernel maintains a special internal data structure for each semaphore
set which exists within its addressing space. This structure is of type <TT>semid_ds</TT>, and
is defined in <TT>linux/sem.h</TT> as follows:
<P>
<P>
<HR><PRE> /* One semid data structure for each set of semaphores in the system. */
struct semid_ds {
struct ipc_perm sem_perm; /* permissions .. see ipc.h */
time_t sem_otime; /* last semop time */
time_t sem_ctime; /* last change time */
struct sem *sem_base; /* ptr to first semaphore in array */
struct wait_queue *eventn;
struct wait_queue *eventz;
struct sem_undo *undo; /* undo requests on this array */
ushort sem_nsems; /* no. of semaphores in array */
};</PRE>
<HR>As with message queues, operations on this structure are performed by a special system call, and
should not be tinkered with directly. Here are descriptions of the more pertinent fields:
<P>
<DL ><DT><STRONG><TT>sem_perm</TT></STRONG>
<DD>
<P>
This is an instance of the <TT>ipc_perm</TT> structure, which is defined
for us in <TT>linux/ipc.h</TT>. This holds the permission information for
the semaphore set, including the access permissions, and information
about the creator of the set (uid, etc).
<P>
<DT><STRONG><TT>sem_otime</TT></STRONG>
<DD>
<P>
Time of the last <TT>semop()</TT> operation (more on this in a moment)
<P>
<DT><STRONG><TT>sem_ctime</TT></STRONG>
<DD>
<P>
Time of the last change to this structure (mode change, etc)
<P>
<DT><STRONG><TT>sem_base</TT></STRONG>
<DD>
<P>
Pointer to the first semaphore in the array (see next structure)
<P>
<DT><STRONG><TT>sem_undo</TT></STRONG>
<DD>
<P>
Number of <EM>undo</EM> requests in this array (more on this in a moment)
<P>
<DT><STRONG><TT>sem_nsems</TT></STRONG>
<DD>
<P>
Number of semaphores in the semaphore set (the array)
<P>
</DL><BR> <HR>
<P><ADDRESS>
<I>Converted on: <BR>
Fri Mar 29 14:43:04 EST 1996</I>
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