In support of tptacek's comment, an abbreviate version of what's in assert.h on my OSX system:

  #ifdef NDEBUG
  #define assert(e)   ((void)0)
  #else

  #define assert(e)  \
    ((void) ((e) ? 0 : __assert (#e, __FILE__, __LINE__)))
  #define __assert(e, file, line) \
    ((void)printf ("%s:%u: failed assertion `%s'\n", file, line, e), abort())

So, if NDEBUG ("no debug") is defined, which it is if optimizations are turned on, asserts become a no-op.