Is it safe to use C function pointer this way?

Question

The source code example is:

int add2(int a0, int a1)
{
    return a0+a1;
}

int add4(int a0, int a1, int a2, int a3)
{
    return a0+a1+a2+a3;
}

int (*fptr)(int a0,int a1,int a2,int a3,int a4,int a5,int a6,int a7);

int main()
{
    volatile int ret0;
    int a0=0;
    int a1=1;
    int a2=2;
    int a3=3;
    int a4=4;
    int a5=5;
    int a6=6;
    int a7=7;

    fptr = add4;
    ret0=fptr(a0,a1,a2,a3,a4,a5,a6,a7);

    return ret0;
}

In this example we have function pointer with 8 arguments. I'am assigning to this pointer the pointer of the function which contains 4 arguments. In this case, compiler may generate assembly code, which will use stack and registers, which are not used in the target function (with 4 arguments). Is it legal to assign and use address of function which has less arguments than the function pointer? Is it safe to use such a programming pattern?

`uncp.c:25:10: warning: assignment from incompatible pointer type [enabled by default] fptr = add4;` I got this warning. — kiran Biradar, Jul 15 '19 at 11:33
Right. I've got the same warning. But it doesn't say it is an error. — lol lol, Jul 15 '19 at 11:35
Why would you do such a thing? You should be using an array instead of defining lots of variables and use one single function, passing in the array and other necessary parameters — Spikatrix, Jul 15 '19 at 11:35
A function call that passes a different number of arguments than those expected is Undefined Behaviour. — alx - recommends codidact, Jul 15 '19 at 11:48
@Cacahuete Why do you think so? Could you please give us the link to the mentioned conclusion? — lol lol, Jul 15 '19 at 11:51
The behaviour is undefined. Which means, essentially, that (1) anything can happen and (2) no diagnostic (e.g. from the compiler) is required. Given those two characteristics, I would say the code is unsafe. I cannot comment on whether *you* would describe it as safe or unsafe. — Peter, Jul 15 '19 at 11:53
Then thank all of you guys! I appreciate your opinion and thanks for information! — lol lol, Jul 15 '19 at 11:57

alx - recommends codidact · Accepted Answer · 2019-07-15T12:50:18.577

The assignment is valid:

fptr = add4;

C17::6.3.2.3::8:

A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer. If a converted pointer is used to call a function whose type is not compatible with the referenced type, the behavior is undefined.

However, it's only valid to store it as a different function pointer, and not valid to use it (actually call the function):

ret0 = fptr(a0, a1, a2, a3, a4, a5, a6, a7);

C17::6.5.2.2::Constraints.2:

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

Links: Where do I find the current C or C++ standard documents?

Rishikesh Raje · Answer 2 · 2019-07-15T11:45:11.607

2

Is it legal to assign and use address of function which has less arguments than the function pointer?

No, the function pointer should have the same type and number of arguments as the underlying function.

Ref http://port70.net/~nsz/c/c99/n1256.html#6.5.16.1

The following constraints shall hold for simple assignment

...
both operands are pointers to qualified or unqualified versions of compatible types, and the type pointed to by the left has all the qualifiers of the type pointed to by the right;
..

edited Jul 15 '19 at 11:45

answered Jul 15 '19 at 11:39

Rishikesh Raje

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No. The Undefined Behaviour is in the call; the assignment is valid: **C17::6.3.2.3::8:** *"A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer. If a converted pointer is used to call a function whose type is not compatible with the referenced type, the behavior is undefined."* – alx - recommends codidact Jul 15 '19 at 12:38

Is it safe to use C function pointer this way?

2 Answers2