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freefal

What does the provided example do?

5) Surprising math:

int x = 0xfffe+0x0001;

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It doesn't compile.

It's trying to parse the e as scientific notation I think.

  surprising_math.c: In function ‘main’:
  surprising_math.c:4:10: error: invalid suffix "+0x0001" on integer constant
    int x = 0xfffe+0x0001;
            ^~~~~~~~~~~~~

Indeed, I think you're exactly right. Changing it to `0xffff+0x0001` lets it compile.

Doesn't that defeat the purpose then? It's no longer using scientific notation.

The initial intent was to add 2 hex values, but using 0xfffe tripped the parser.

It looks a bit like a binary exponent, but according to clang and gcc it's erroneous: 'invalid suffix '+0x0001' on integer constant'.

That is really stupid. How is that not a bug? I can't belive GCC wont compile it. What does other compilers do.

Edit: Clang also gives an error. Mscv seems to compile. I wonder who follows spec. I assume not mscv ...

It's not a bug. 0xfffe+0x0001 satisfies the syntax of a "preprocessing number". In a later translation phase, preprocessing numbers are converted to numeric constants. Not all valid preprocessing numbers are valid numeric constants. The syntax of preprocessing numbers is deliberately permissive to avoid complicating the implementation of the preprocessor.

I'm sure it could have been made stricter, allowing 0xfffe+0x0001 to be treated as 0xfffe + 0x0001 -- but the solution is simply to write 0xfffe + 0x0001 in the first place. The language grammar has to be consistently defined even if it leads to surprises now and then.

It produces an error message.

error: invalid suffix '+0x0001' on integer constant

[0] https://gcc.godbolt.org/z/pR5L6v

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