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DannyNiu
DannyNiu

Reputation: 1487

Integer type for holding function pointers?

What standard-defined integer type should I use for holding pointer to functions? Is there a (void*)-like type for functions that can hold any functions?

It's very certain that it's not [u]intptr_t because the standard said explicitly it's for pointers to objects and the standard makes a clear distinction between pointer to objects and pointer to functions.

Upvotes: 3

Views: 216

Answers (2)

DannyNiu
DannyNiu

Reputation: 1487

It's very certain that it's not [u]intptr_t

It's true. But the distinction is only made to guarantee the correctness of some essential conversions. If you look at the 2nd edition of "The C Programming Language", then the distinction is more subtle to notice than the current wording in the standard.

If you target platforms with operating systems, then [u]intptr_t is probably exactly what you want, as:

  1. POSIX specifies dlsym function that returns void * with the requirement that result can be casted to function pointers and be usable.

  2. Win32 GetProcAddress is defined in such way that, if the return values' not NULL, it'll be a valid memory address that's valid for functions and/or objects.

Upvotes: 2

chux
chux

Reputation: 153447

There is no specified type for an integer type that is sufficient to encode a function pointer.

Alternatives:

  • Change code to negate the need for that integer type. Rarely is such an integer type truly needed.

  • Use an array: unsigned char[sizeof( int (*)(int)) )] and int (*f)(int)) within union to allow some examination of the integer-ness of the pointer. Still there may be padding issues. Comes down to what code want to do with such an integer.

  • Use uintmax_t and hope it is sufficient. A _Static_assert(sizeof (uintmax_t) >= sizeof (int (*)(int)) ); is a reasonable precaution though not a guarantee of success.

The limiting spec

Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type. C17dr § 6.3.2.3 6

Note even [u]intptr_t for object pointer types are a guarantee either as they are optional types.

Upvotes: 3

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