int __builtin_constant_p (exp)函数的使用

 int __builtin_constant_p (exp)是GCC内置函数

功能:

— Built-in Function: int __builtin_constant_p (exp)

You can use the built-in function __builtin_constant_p todetermine if a value is known to be constant at compile time and hencethat GCC can perform constant-folding on expressions involving thatvalue. The argument of the function is the value to test. The functionreturns the integer 1 if the argument is known to be a compile-timeconstant and 0 if it is not known to be a compile-time constant. Areturn of 0 does not indicate that the value isnot a constant,but merely that GCC cannot prove it is a constant with the specifiedvalue of the-O option.

也就是说在编译时，只要exp参数是常数，就返回1，否则返回0.