CODEWITHSUNDEEP
cpointers
emandret
emandret

Reputation: 1399

Function pointer arithmetic

Why is this code compiling correctly for the arithmetic on a function pointer?

void my_func(void);

int main(void)
{
    void (*p)(void) = &my_func;

    // Compile
    (void) (*p);
    (void) *(p + 0);

    // Does not compile
    (void) p[0];

    return 0;
}

I have always thought that p[0] == *(p + 0) for complete types. Apparently, p[0] != *(p + 0) for function pointers.

Note: the C standard does not explicitly forbid function pointer arithmetic. It does not forbid it at all. It says it is undefined. That is a different thing. Many language extensions that are conforming in the terms of the standard have behavior that is undefined by the standard.

Also, if you use a pointer to an incomplete type, then we have:

int main(void)
{
    int (*p)[];

    // Compile
    (void) (*p);

    // Does not compile
    (void) *(p + 0);
    (void) p[0];

    return 0;
}

Then effectively p[0] == *(p + 0) because both triggers the same error for arithmetic on an pointer to an incomplete type. Although here, the C standard does explicitly forbid arithmetic on an pointer to an incomplete type.

Upvotes: 0

Views: 379

Answers (1)

Paul Hankin
Paul Hankin

Reputation: 58339

Array subscripting a[b] needs one of a or b to be a pointer to a complete object type. A function type is not a complete object type.

6.5.2.1 Array subscripting

Constraints

1 One of the expressions shall have type "pointer to complete object type", the other expression shall have integer type, and the result has type "type".

Note that p + 0 should also be an error for the same reason, and I believe the compiler is failing to produce a required diagnostic message. I asked this about why it's not produced: Should clang and gcc produce a diagnostic message when a program does pointer arithmetic on a function pointer?

6.5.6 Additive operators

Constraints

For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to a complete object type and the other shall have integer type. (Incrementing is equivalent to adding 1.)

Upvotes: 1

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