projects / OpenSPARC-T2-DV / blame
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| 54 | ||
| 55 | <H2><A NAME="SECTION009540000000000000000"></A><A NAME="semaphore-objects"></A> | |
| 56 | <BR> | |
| 57 | 7.5.4 Semaphore Objects | |
| 58 | </H2> | |
| 59 | ||
| 60 | <P> | |
| 61 | This is one of the oldest synchronization primitives in the history of | |
| 62 | computer science, invented by the early Dutch computer scientist | |
| 63 | Edsger W. Dijkstra (he used <tt class="method">P()</tt> and <tt class="method">V()</tt> instead of | |
| 64 | <tt class="method">acquire()</tt> and <tt class="method">release()</tt>). | |
| 65 | ||
| 66 | <P> | |
| 67 | A semaphore manages an internal counter which is decremented by each | |
| 68 | <tt class="method">acquire()</tt> call and incremented by each <tt class="method">release()</tt> | |
| 69 | call. The counter can never go below zero; when <tt class="method">acquire()</tt> | |
| 70 | finds that it is zero, it blocks, waiting until some other thread | |
| 71 | calls <tt class="method">release()</tt>. | |
| 72 | ||
| 73 | <P> | |
| 74 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 75 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-2730' xml:id='l2h-2730' class="class">Semaphore</tt></b>(</nobr></td> | |
| 76 | <td><var></var><big>[</big><var>value</var><big>]</big><var></var>)</td></tr></table></dt> | |
| 77 | <dd> | |
| 78 | The optional argument gives the initial value for the internal | |
| 79 | counter; it defaults to <code>1</code>. | |
| 80 | </dl> | |
| 81 | ||
| 82 | <P> | |
| 83 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 84 | <td><nobr><b><tt id='l2h-2731' xml:id='l2h-2731' class="method">acquire</tt></b>(</nobr></td> | |
| 85 | <td><var></var><big>[</big><var>blocking</var><big>]</big><var></var>)</td></tr></table></dt> | |
| 86 | <dd> | |
| 87 | Acquire a semaphore. | |
| 88 | ||
| 89 | <P> | |
| 90 | When invoked without arguments: if the internal counter is larger than | |
| 91 | zero on entry, decrement it by one and return immediately. If it is | |
| 92 | zero on entry, block, waiting until some other thread has called | |
| 93 | <tt class="method">release()</tt> to make it larger than zero. This is done with | |
| 94 | proper interlocking so that if multiple <tt class="method">acquire()</tt> calls are | |
| 95 | blocked, <tt class="method">release()</tt> will wake exactly one of them up. The | |
| 96 | implementation may pick one at random, so the order in which blocked | |
| 97 | threads are awakened should not be relied on. There is no return | |
| 98 | value in this case. | |
| 99 | ||
| 100 | <P> | |
| 101 | When invoked with <var>blocking</var> set to true, do the same thing as | |
| 102 | when called without arguments, and return true. | |
| 103 | ||
| 104 | <P> | |
| 105 | When invoked with <var>blocking</var> set to false, do not block. If a | |
| 106 | call without an argument would block, return false immediately; | |
| 107 | otherwise, do the same thing as when called without arguments, and | |
| 108 | return true. | |
| 109 | </dl> | |
| 110 | ||
| 111 | <P> | |
| 112 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 113 | <td><nobr><b><tt id='l2h-2732' xml:id='l2h-2732' class="method">release</tt></b>(</nobr></td> | |
| 114 | <td><var></var>)</td></tr></table></dt> | |
| 115 | <dd> | |
| 116 | Release a semaphore, | |
| 117 | incrementing the internal counter by one. When it was zero on | |
| 118 | entry and another thread is waiting for it to become larger | |
| 119 | than zero again, wake up that thread. | |
| 120 | </dl> | |
| 121 | ||
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| 127 | <A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a> | |
| 128 | ||
| 129 | <UL CLASS="ChildLinks"> | |
| 130 | <LI><A href="semaphore-examples.html">7.5.4.1 <tt class="class">Semaphore</tt> Example</a> | |
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| 169 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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