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<TITLE>SYSTEM CALL: semop()</TITLE>
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<B> Next:</B> <A NAME="tex2html1025" HREF="node53.html">SYSTEM CALL: semctl()</A>
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<B>Up:</B> <A NAME="tex2html1023" HREF="node46.html">6.4.3 Semaphores</A>
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<B> Previous:</B> <A NAME="tex2html1017" HREF="node51.html">SYSTEM CALL: semget()</A>
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<BR> <P>
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<H3><A NAME="SECTION00743400000000000000">SYSTEM CALL: semop()</A></H3>
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<P>
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<P>
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<HR><PRE> SYSTEM CALL: semop();
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PROTOTYPE: int semop ( int semid, struct sembuf *sops, unsigned nsops);
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RETURNS: 0 on success (all operations performed)
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-1 on error: errno = E2BIG (nsops greater than max number of ops allowed atomically)
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EACCESS (permission denied)
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EAGAIN (IPC_NOWAIT asserted, operation could not go through)
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EFAULT (invalid address pointed to by sops argument)
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EIDRM (semaphore set was removed)
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EINTR (Signal received while sleeping)
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EINVAL (set doesn't exist, or semid is invalid)
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ENOMEM (SEM_UNDO asserted, not enough memory to create the
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undo structure necessary)
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ERANGE (semaphore value out of range)
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NOTES:</PRE>
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<HR>The first argument to <TT>semget()</TT> is the key value (in our case returned by a call
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to <TT>semget</TT>). The second argument (<TT>sops</TT>) is a pointer to an array of <EM>operations</EM> to
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be performed on the semaphore set, while the third argument (<TT>nsops</TT>) is the number
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of operations in that array.
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<P>
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The <TT>sops</TT> argument points to an array of type <TT>sembuf</TT>. This structure is
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declared in <TT>linux/sem.h</TT> as follows:
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<P>
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<P>
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<HR><PRE> /* semop system call takes an array of these */
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struct sembuf {
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ushort sem_num; /* semaphore index in array */
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short sem_op; /* semaphore operation */
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short sem_flg; /* operation flags */
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};</PRE>
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<HR><DL ><DT><STRONG><TT>sem_num</TT></STRONG>
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<DD>
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<P>
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The number of the semaphore you wish to deal with
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<P>
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<DT><STRONG><TT>sem_op</TT></STRONG>
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<DD>
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<P>
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The operation to perform (positive, negative, or zero)
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<P>
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<DT><STRONG><TT>sem_flg</TT></STRONG>
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<DD>
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<P>
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Operational flags
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<P>
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</DL>
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<P>
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If <TT>sem_op</TT> is negative, then its value is subtracted from the semaphore.
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This correlates with obtaining resources that the semaphore controls or monitors access of.
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If <B>IPC_NOWAIT</B> is not specified, then the calling process sleeps until the requested amount of
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resources are available in the semaphore (another process has released some).
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<P>
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If <TT>sem_op</TT> is positive, then it's value is added to the semaphore.
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This correlates with returning resources back to the application's semaphore set.
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Resources should always be returned to a semaphore set when they are no longer needed!
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<P>
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Finally, if <TT>sem_op</TT> is zero (0), then the calling process will sleep() until the
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semaphore's value is 0. This correlates to waiting for a semaphore to reach 100% utilization.
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A good example of this would be a daemon running with superuser permissions that could
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dynamically adjust the size of the semaphore set if it reaches full utilization.
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<P>
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In order to explain the <TT>semop</TT> call, let's revisit our print room scenario. Let's
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assume only one printer, capable of only one job at a time. We create a semaphore set with
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only one semaphore in it (only one printer), and initialize that one semaphore to a value of one (only
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one job at a time).
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<P>
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Each time we desire to send a job to this printer, we need to first make sure that the resource
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is available. We do this by attempting to obtain one <EM>unit</EM> from the semaphore. Let's
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load up a sembuf array to perform the operation:
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<P>
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<P>
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<HR><PRE> struct sembuf sem_lock = { 0, -1, IPC_NOWAIT };</PRE>
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<HR>Translation of the above initialized structure dictates that a value of ``-1'' will be added
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to semaphore number 0 in the semaphore set. In other words, one unit of resources will be
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obtained from the only semaphore in our set (0th member). <B>IPC_NOWAIT</B> is specified,
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so the call will either go through immediately, or fail if another print job is currently
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printing. Here is an example of using this initialized <TT>sembuf</TT> structure with the <TT>semop</TT>
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system call:
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<P>
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<P>
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<HR><PRE> if((semop(sid, &sem_lock, 1) == -1)
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perror("semop");</PRE>
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<HR>The third argument (<TT>nsops</TT>) says that we are only performing one (1) operation (there
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is only one <TT>sembuf</TT> structure in our array of operations). The <TT>sid</TT> argument
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is the IPC identifier for our semaphore set.
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<P>
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When our print job has completed, we must <EM>return</EM> the resources back to the semaphore
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set, so that others may use the printer.
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<P>
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<P>
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<HR><PRE> struct sembuf sem_unlock = { 0, 1, IPC_NOWAIT };</PRE>
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<HR>Translation of the above initialized structure dictates that a value of ``1'' will be added to
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semaphore number 0 in the semaphore set. In other words, one unit of resources will be returned
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to the set.
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<P>
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<HR><A NAME="tex2html1024" HREF="node53.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A> <A NAME="tex2html1022" HREF="node46.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A> <A NAME="tex2html1016" HREF="node51.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <A NAME="tex2html1026" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME="tex2html1025" HREF="node53.html">SYSTEM CALL: semctl()</A>
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<B>Up:</B> <A NAME="tex2html1023" HREF="node46.html">6.4.3 Semaphores</A>
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<B> Previous:</B> <A NAME="tex2html1017" HREF="node51.html">SYSTEM CALL: semget()</A>
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<P><ADDRESS>
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<I>Converted on: <BR>
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Fri Mar 29 14:43:04 EST 1996</I>
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