According to the language definition, a constant 0 in a pointer context is converted into a null pointer at compile time.
That is, in an initialization, assignment, or comparison when one side is a variable or expression of pointer type, the compiler can tell that a constant 0 on the other side requests a null pointer, and generate the correctly-typed null pointer value.
Therefore, the following fragments are perfectly legal:

char *p = 0;
if(p != 0)

However, an argument being passed to a function is not necessarily recognizable as a pointer context, and the compiler may not be able to tell that an unadorned 0 "means" a null pointer.
To generate a null pointer in a function call context, an explicit cast may be required, to force the 0 to be recognized as a pointer.

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