Thread: writing an interpreter for this programming language - segmentation error

This is not a problem. A plain 0 in a pointer context is treated as a null pointer. Other equivalent forms are treated the same way, such as the expression (!file) which is equivalent to (file != 0), which is a valid comparison against a null pointer.

I did say that it might not be a problem - I was just saying that the value of NULL is implementation defined; it doesn't have to be zero.

References
C11 - 7.19 #3
C99 - 7.17 #3
C89 - 4.1.5

I was shocked as you when I learnt that I was making non-portable code by looking for !fp -

Both forms are valid (and portable) to compare the file pointer against a null pointer

From all three standards

C11
NULL
which expands to an implementation-defined null pointer constant

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I've found something that will fix the confusion - I was wrong, because the null pointer constant is defined as

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

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