writing an interpreter for this programming language - segmentation error

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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.

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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 -

I made a typo in my last post: (!file) is equivalent to (file == 0), NOT to (file != 0).

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

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

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From all three standards

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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

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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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Using ASCII vs numbers does not have any performance impact. They are compiled to the exact same machine code.

Even if it does, don't optimize prematurely. Don't optimize if you don't need to. Don't optimize unless the code is too slow, and you know exactly which part is too slow.

In 99% of cases, code clarity is much more important.