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NULL

NULL is the pointer to nothing. It evaluates to 0, and is always
false in a boolean context.

You can never dereference NULL. It is a run-time error at least,
and machine-crasher at worst.

Sometimes you see people use 0 as NULL, but don't.

char *p = NULL; /* good */
char *p = 0;    /* compiles the same, but less clear */
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