Eisenberg & McGuire algorithm

<h2 id="algorithm">Algorithm</h2>
<p>All the <i>n</i>-processes share the following variables:
</p>
enum pstate = {IDLE, WAITING, ACTIVE};
pstate flags[n];
int turn;

<p>The variable turn is set arbitrarily to a number between 0 and <i>n</i>−1 at the start of the <a href="/facts/Algorithm/fnl5NmRt">algorithm</a>.
</p><p>The flags variable for each process is set to WAITING whenever it intends to enter the <a href="/facts/Critical_section/zYwj5uh5">critical section</a>. flags takes either IDLE or WAITING or ACTIVE.
</p><p>Initially the flags variable for each process is initialized to IDLE.
</p>
    repeat {

/* announce that we need the resource */
		flags[i] := WAITING;

/* scan processes from the one with the turn up to ourselves. */
		/* repeat if necessary until the scan finds all processes idle */
		index := turn;
		while (index != i) {
			if (flags[index] != IDLE) index := turn;
			else index := (index+1) mod n;
		}

/* now tentatively claim the resource */
		flags[i] := ACTIVE;

/* find the first active process besides ourselves, if any */
		index := 0;
		while ((index < n) && ((index = i) || (flags[index] != ACTIVE))) {
			index := index+1;
		}

/* if there were no other active processes, AND if we have the turn
	   or else whoever has it is idle, then proceed.  Otherwise, repeat
	   the whole sequence. */
    } until ((index >= n) && ((turn = i) || (flags[turn] = IDLE)));

/* Start of CRITICAL SECTION */

/* claim the turn and proceed */
	turn := i;

/* Critical Section Code of the Process */

/* End of CRITICAL SECTION */

/* find a process which is not IDLE */
	/* (if there are no others, we will find ourselves) */
	index := (turn+1) mod n;
	while (flags[index] = IDLE) {
		index := (index+1) mod n;
	}

/* give the turn to someone that needs it, or keep it */
	turn := index;

/* we're finished now */
	flags[i] := IDLE;

/* REMAINDER Section */

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Dekker%27s_algorithm/xfRMh5hI">Dekker's algorithm</a></li>
<li><a href="/facts/Peterson%27s_algorithm/DU3jLWsk">Peterson's algorithm</a></li>
<li><a href="/facts/Lamport%27s_bakery_algorithm/LhNIQ02p">Lamport's bakery algorithm</a></li>
<li><a href="/facts/Szyma%C5%84ski%27s_algorithm/HxtQlRLl">Szymański's algorithm</a></li>
<li><a href="/facts/Semaphore_(programming)/BrE1W4Je">Semaphores</a></li></ul>

<ul><li><a href="http://portal.acm.org/citation.cfm?id=361895">http://portal.acm.org/citation.cfm?id=361895</a></li></ul>
<h2 id="external-links">External links</h2>
<ul><li><a href="https://web.archive.org/web/20120617015556/http://lcsee.wvu.edu/~jdmooney/classes/cs550/notes/tech/mutex/Eisenberg.html">https://web.archive.org/web/20120617015556/http://lcsee.wvu.edu/~jdmooney/classes/cs550/notes/tech/mutex/Eisenberg.html</a></li>
<li><a href="http://www.cs.csustan.edu/~john/Classes/Previous_Semesters/CS3750_OperatingSys_I/1999_04_Fall/Notes/nProcessSynch.html">http://www.cs.csustan.edu/~john/Classes/Previous_Semesters/CS3750_OperatingSys_I/1999_04_Fall/Notes/nProcessSynch.html</a></li></ul>

Eisenberg & McGuire algorithm open-in-new

Eisenberg & McGuire algorithm