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I'm not sure pointer arithmetic on function pointers is standard C. Using the -pedantic flag, gcc 4.5.2 does give a "warning: pointer to a function used in subtraction".

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The paragraph on pointer differences (ANSI C99 6.5.6§9) states that : "both shall point to elements of the same array object, or one past the last element of the array object […] The size of the result is implementation-defined, and its type (a signed integer type) is ptrdiff_t".

So, for an array of function pointer that would be legal behaviour, but for pointers to plain functions, it is undefined behaviour.

When I was in high school I thought I could implement shared libraries for DOS/DJGPP by subtracting function pointers to each function I wanted to export and dumping the intervening data to disk. I quickly realized that the code was not position independent, and that other segments (like strings) were not being exported. I also discovered Linux, and thus abandoned the idea.

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