What does (~0L) mean?

问题

I'm doing some X11 ctypes coding, I don't know C but need some help understanding this.

In the C code below (might be C++ im not sure) we see (~0L) what does that mean? In Javascript and Python ~0 means -1.

812   int result = GetProperty(window, property_name,
813                            (~0L), // (all of them)
814                            &type, &format, &num_items, &properties);

Thanks

回答1:

0L is a long integer value with all the bits set to zero - that's generally the definition of 0. The ~ means to invert all the bits, which leaves you with a long integer with all the bits set to one.

In two's complement arithmetic (which is almost universal) a signed value with all bits set to one is -1.

The reason for using ~0L instead of -1L is to be clearer about the intent - it's not meant to be used as a number at all, but rather as a collection of bits.

回答2:

Bitwise compliment of zero of long type.

来源：https://stackoverflow.com/questions/27596116/what-does-0l-mean