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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
787
0L, abbreviation for 0 longitude, or the Prime meridian.
The 0L means the number zero of type long . It uses this constructor to instantiate a Date that refers to zero milliseconds after (i.e. exactly) "the epoch", January 1, 1970, 00:00:00 GMT.
The L specifies that the number is a long type, so -1L is a long set to negative one, and 1L is a long set to positive one. As for why ftell doesn't just return NULL , it's because NULL is used for pointers, and here a long is returned. Note that 0 isn't used because 0 is a valid value for ftell to return.
(long) 1 is a constant expression (because the 1 is directly known) and hence, by the rules of the Java Language Specification (JLS), will be a long after compilation already. However, in my experience, it is far more common that people use the long literal and write 1L for readability.
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.
144
Bitwise compliment of zero of long type.
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