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POWhat exactly is a reentrant function?
 

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  1. POWhat exactly is a reentrant function?
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    <p><a href="https://stackoverflow.com/questions/1435541/is-this-function-reentrant/1435561#1435561">Most</a> <a href="https://stackoverflow.com/questions/856823/threadsafe-vs-re-entrant/856856#856856">of</a> <a href="https://stackoverflow.com/questions/1435541/is-this-function-reentrant/1435577#1435577">the</a> <a href="https://stackoverflow.com/questions/2441351/what-is-a-re-entrant-parser/2441401#2441401">times</a>, the definition of reentrance is quoted from <a href="http://en.wikipedia.org/wiki/Reentrant_%28subroutine%29" rel="noreferrer">Wikipedia</a>:</p> <blockquote> <p>A computer program or routine is described as reentrant if it can be <strong><em>safely</em></strong> called again before its previous invocation has been completed (i.e it can be safely executed concurrently). To be reentrant, a computer program or routine:</p> <ol> <li>Must hold no static (or global) non-constant data. </li> <li>Must not return the address to static (or global) non-constant data. </li> <li>Must work only on the data provided to it by the caller. </li> <li>Must not rely on locks to singleton resources. </li> <li>Must not modify its own code (unless executing in its own unique thread storage) </li> <li>Must not call non-reentrant computer programs or routines.</li> </ol> </blockquote> <p>How is <strong><em>safely</em></strong> defined? </p> <p>If a program can be <strong><em>safely executed concurrently</em></strong>, does it always mean that it is reentrant?</p> <p>What exactly is the common thread between the six points mentioned that I should keep in mind while checking my code for reentrant capabilities?</p> <p>Also,</p> <ol> <li>Are all recursive functions reentrant?</li> <li>Are all thread-safe functions reentrant?</li> <li>Are all recursive and thread-safe functions reentrant?</li> </ol> <hr> <p>While writing this question, one thing comes to mind: Are the terms like <strong>reentrance</strong> and <strong>thread safety</strong> absolute at all i.e. do they have fixed concrete definations? For, if they are not, this question is not very meaningful.</p>
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    1. COActually, I disagree with #2 in the first list. You can return a address to whatever you like from a re-entrant function - the limitation is on what you do with that address in the calling code.
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    2. CO@Neil But as the writer of the reentrant function can't control what the caller surely surely they must not return an address to static (or global) non-constant data for it to be truely reentrant?
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    3. CO@drelihan It is not the responsibility of the writer of ANY function (reentrant or not) to control what a caller does with a returned value. They should certainly say what the caller CAN do with it, but if the caller chooses to do something else - tough luck to the caller.
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