On 19 Jun 2008, at 4:16 am, Anatoly Yakovenko wrote:
C doesn't work like that :). functions always get called.
Not true. A C compiler must produce the same *effect* as if the function had been called, but if by some means the compiler knows that the function has no effect, it is entitled to skip the call. In particular, the C compiler I normally use offers these pragmas, amongst others: #pragma does_not_write_global_data (funcname [, funcname]) #pragma no_side_effect(funcname[, funcname]) So with a declaration like extern double cblas_ddot( int, double const *, int, double const *, int); #pragma no_side_effect (cblas_ddot)
the compiler would be completely within its rights to discard any call to cblas_ddot() whose result was not used. (As it happens, it didn't, but it would have been allowed to.) If using gcc, extern double cblas_ddot( ... as before ...) __attribute__ ((const)); seems to have the same effect, certainly the test case I tried did in fact completely eliminate a call to cblas_ddot() when so declared. Since the malloc() results pointed to uninitialised memory, the C compiler was entitled to do anything it pleased anyway.