sizeof對於a bit-field, a void expression, or a function designator無意義

看下C標準吧
C89 specified that sizeof’s operand can be any value except a bit-field, a void expression, or a function designator.
---所以對函數本身的sizeof應該是無意義的
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these expressions determine the size of the type used for:
sizeof(F(x)) // ... F’s return value
----sizeof(F(x))可以得到函數返回值的大小