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    <p>Re-entrant functions do not rely on global variables that are exposed in the C library headers .. take strtok() vs strtok_r() for example in C.</p> <p>Some functions need a place to store a 'work in progress' , re-entrant functions allow you to specify this pointer within the thread's own storage, not in a global. Since this storage is exclusive to the calling function, it can be interrupted and <em>re-entered</em> (re-entrant) and since in most cases mutual exclusion beyond what the function implements isn't required for this to work, they are often considered to be <em>thread safe</em>. This isn't, however, guaranteed by definition. </p> <p>errno, however, is a slightly different case on POSIX systems (and tends to be the oddball in any explanation of how this all works) :)</p> <p>In short, reentrant <em>often</em> means thread safe (as in "use the reentrant version of that function if you're using threads"), but thread safe does not always mean re-entrant (or the reverse). When you're looking at thread-safety, <em>concurrency</em> is what you need to be thinking about. If you have to provide a means of locking and mutual exclusion to use a function, then the function isn't inherently thread-safe. </p> <p>But, not all functions need to be examined for either. <code>malloc()</code> has no need to be reentrant, it does not depend on anything out of the scope of the entry point for any given thread (and is itself thread safe). </p> <p>Functions that return statically allocated values are <em>not</em> thread safe without the use of a mutex, futex, or other atomic locking mechanism. Yet, they don't need to be reentrant if they're not going to be interrupted. </p> <p>i.e.:</p> <pre><code>static char *foo(unsigned int flags) { static char ret[2] = { 0 }; if (flags &amp; FOO_BAR) ret[0] = 'c'; else if (flags &amp; BAR_FOO) ret[0] = 'd'; else ret[0] = 'e'; ret[1] = 'A'; return ret; } </code></pre> <p>So, as you can see, having multiple threads use that without some kind of locking would be a disaster .. but it has no purpose being re-entrant. You'll run into that when dynamically allocated memory is taboo on some embedded platform.</p> <p>In purely functional programming, reentrant often <em>doesn't</em> imply thread safe, it would depend on the behavior of defined or anonymous functions passed to the function entry point, recursion, etc. </p> <p>A better way to put 'thread safe' is <strong><em>safe for concurrent access</em></strong> , which better illustrates the need. </p>
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    1. COReentrant does not imply thread-safe. Pure functions imply thread-safety.
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