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In computer programming jargon, a heisenbug is a software bug that seems to disappear or alter its behavior when one attempts to study it. The term is a pun on the name of Werner Heisenberg, the physicist who first asserted the observer effect of quantum mechanics, which states that the act of observing a system inevitably alters its state. In electronics the traditional term is probe effect, where attaching a test probe to a device changes its behavior. Similar terms, such as bohrbug, mandelbug,〔Raymond, Eric S.; (''The New Hacker's Dictionary'' ), 3rd edition, 1996〕〔Clarke, Arthur C., (''The Ghost from the Grand Banks'' ), Bantam Books, 1990〕 and schrödinbug〔Raymond, Eric S.; (''The New Hacker's Dictionary'' ), 3rd edition, 1996〕 have been occasionally proposed for other kinds of unusual software bugs, sometimes in jest;〔The following article investigates the various definitions of bohrbug, mandelbug and heisenbug proposed in the literature, as well as the statements made about the relationships between these fault types: Grottke, Michael; and Trivedi, Kishor S.; ''Software Faults, Software Aging and Software Rejuvenation'', Journal of the Reliability Engineering Association of Japan, Vol. 27, No. 7, pp. 425-438, 2005.〕〔Grottke, Michael; and Trivedi, Kishor S.; (''Fighting Bugs: Remove, Retry, Replicate, and Rejuvenate'' ), IEEE Computer vol. 40, no. 2 (February 2007), pp. 107-109〕 however, unlike "heisenbug", they are not widely known or used.〔A February 2012 Google Books search returns about 70 hits for "schroedinbug", 100 for "mandelbug", 400 for "bohrbug" or "heisenbug".〕 ==Examples== Heisenbugs occur because common attempts to debug a program, such as inserting output statements or running it in a debugger, usually modify the code, change the memory addresses of variables and the timing of its execution. One common example of a heisenbug is a bug that appears when the program is compiled with an optimizing compiler, but not when the same program is compiled without optimization (as is often done for the purpose of examining it with a debugger). While debugging, values that an optimized program would normally keep in registers are often pushed to main memory. This may affect, for instance, the result of floating-point comparisons, since the value in memory may have smaller range and accuracy than the value in the register. Similarly, Heisenbugs may be caused by side-effects in test expressions used in runtime assertions in languages such as C and C++, where the test expression is not evaluated when assertions are turned off in production code using the NDEBUG macro.Other common causes of heisenbugs are using the value of a non-initialized variable (which may change its address and/or initial value during debugging), or following an invalid pointer (which may point to a different place when debugging). Debuggers also commonly provide watches or other user interfaces that cause additional source code (such as property accessors) to be executed stealthily, which can, in turn, change the state of the program. 〔("Java toString() override with initialization as a side effect" )〕 Time can also be a factor in heisenbugs, particularly with multi-threaded applications. Executing a program under control of a debugger can change the execution timing of the program as compared to normal execution. Time-sensitive bugs such as race conditions may not occur when the program is slowed down by single-stepping source lines in the debugger. This is particularly true when the behavior involves interaction with an entity not under the control of a debugger, such as when debugging network packet processing between two machines and only one is under debugger control. Heisenbugs can be viewed as instances of the observer effect in information technology. Frustrated programmers may humorously blame a heisenbug on the phase of the moon,〔CATB.org, ("phase of the moon" )〕 or (if it has occurred only once) may explain it away as a soft error due to alpha particles or cosmic rays affecting the hardware. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heisenbug」の詳細全文を読む スポンサード リンク
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