在C中比较整数的结果

Question

如果x和y都是int ， xy < 0总是会返回与x < y相同的结果吗？

Answer 1

No. If xy causes overflow or underflow, behavior is undefined (because int is a signed type).

For example INT_MIN - 1 < 0 is undefined behavior, whereas INT_MIN < 1 is defined (and true).

When there's no overflow, then the two expressions, xy < 0 and x < y are the same.

Because compiled code may do whatever it likes when there's undefined behavior, the C compiler is allowed to rewrite xy < 0 as x < y if it wishes. This isn't true if x and y are unsigned types, where overflow is well-defined, and xy < 0 and x < y are not equivalent.

Answer 2

As @sgar91 said, No.

For example:

X=0x80000000 //which is IntMin Y=1 xy < 0 // will be false as xy = 0x7FFFFFFF = +Maxint
but

x < y //will be true