`pure'
     Many functions have no effects except the return value and their
     return value depends only on the parameters and/or global
     variables.  Such a function can be subject to common subexpression
     elimination and loop optimization just as an arithmetic operator
     would be.  These functions should be declared with the attribute
     `pure'.  For example,

          int square (int) __attribute__ ((pure));

     says that the hypothetical function `square' is safe to call fewer
     times than the program says.

     Some of common examples of pure functions are `strlen' or `memcmp'.
     Interesting non-pure functions are functions with infinite loops
     or those depending on volatile memory or other system resource,
     that may change between two consecutive calls (such as `feof' in a
     multithreading environment).

     The attribute `pure' is not implemented in GCC versions earlier
     than 2.96.