How to use static assert in C to check the types of parameters passed to a macro

Question

I need to write a C macro that checks to ensure all parameters passed to it are unsigned and of the same integer type. Ex: all input params are uint8_t

Answer 1

If the most important aspect here is that you want it to fail to compile if a and b are different types, you can make use of C11's _Generic along with GCC's __typeof__ extension to manage this.

A generic example:

#include <stdio.h>

#define TYPE_ASSERT(X,Y) _Generic ((Y), \
    __typeof__(X): _Generic ((X), \
        __typeof__(Y): (void)NULL \
    ) \
)

int main(void)
{
    int a = 1; 
    int b = 2;
    TYPE_ASSERT(a,b);
    printf("a = %d, b = %d\n", a, b);
}

Now if we try to compile this code, it will compile fine and everybody is happy.

If we change the type of b to unsigned int, however, it will fail to compile.

This works because _Generic selection uses the type of a controlling expression ((Y) in this case) to select a rule to follow and insert code corresponding to the rule. In this case, we only provided a rule for __typeof__(X), thus if (X) is not a compatible type for (Y), there is no suitable rule to select and therefore cannot compile. To handle arrays, which have a controlling expression that will decay to a pointer, I added another _Generic that goes the other way ensuring they must both be compatible with one another rather than accepting one-way compatibility. And since--as far as I particularly cared--we only wanted to make sure it would fail to compile on a mismatch, rather than execute something particular upon a match, I gave the corresponding rule the task of doing nothing: (void)NULL

There is a corner case where this technique stumbles: _Generic does not handle Variably Modifiable types since it is handled at compile time. So if you attempt to do this with a Variable Length Array, it will fail to compile.

To handle your specific use-case for fixed-width unsigned types, we can modify the nested _Generic to handle that rather than handling the pecularities of an array:

#define TYPE_ASSERT(X,Y) _Generic ((Y), \
    __typeof__(X): _Generic ((Y), \
        uint8_t: (void)NULL, \
        uint16_t: (void)NULL, \
        uint32_t: (void)NULL, \
        uint64_t: (void)NULL \
   ) \
)

Example GCC error when passing non-compatible types:

main.c: In function 'main':
main.c:7:34: error: '_Generic' selector of type 'signed char' is not compatible with any association
    7 |         __typeof__(X): _Generic ((Y), \
      |                                  ^

It is worth mentioning that __typeof__, being a GCC extension, will not be a solution that is portable to all compilers. It does seem to work with Clang, though, so that's another major compiler supporting it.

Answer 2

What you want is doable in standard C11, no extensions or GCC required.

We'll build up to the final answer, so all can follow.

---

According to the C11 standard [6.7.10], static_assert-declaration: _Static_assert( constant-expression , string-literal ) is a Declaration. Thus if we are going to use a macro, we had best provide a scope for a declaration, to keep things tidy. Typically of the usual form:

#define MY_AMAZING_MACRO() do {_Static_assert(...some magic...);} while(0)

---

Next, so that our _Static_assert within the macro at least repeats via stdio the actual issue if the assert fails, well use familiar stringification setup:

#define STATIC_ASSERT_H(x)  _Static_assert(x, #x)
#define STATIC_ASSERT(x)    STATIC_ASSERT_H(x)

---

Next, we'll use C11's Generic selection feature to declare a macro that returns a constant 1 if the object is of the type we're looking for, and zero otherwise:

#define OBJ_IS_OF_TYPE(Type, Obj) _Generic(Obj, Type: 1, default: 0)

---

Next we''l make a macro to test if all four of your inputs are of the same type:

#define ALL_OBJS_ARE_OF_TYPE(Type, Obj_0, Obj_1, Obj_2, Obj_3)  \
    (OBJ_IS_OF_TYPE(Type, Obj_0) &&                             \
     OBJ_IS_OF_TYPE(Type, Obj_1) &&                             \
     OBJ_IS_OF_TYPE(Type, Obj_2) &&                             \
     OBJ_IS_OF_TYPE(Type, Obj_3))

---

Next, using the above, well make a macro to test if all four of your inputs are further one of the four types:

#define IS_ACCEPTABLE(Type_0, Type_1, Type_2, Type_3, Obj_0, Obj_1, Obj_2, Obj_3)   \
    (ALL_OBJS_ARE_OF_TYPE(Type_0, Obj_0, Obj_1, Obj_2, Obj_3) ||                    \
     ALL_OBJS_ARE_OF_TYPE(Type_1, Obj_0, Obj_1, Obj_2, Obj_3) ||                    \
     ALL_OBJS_ARE_OF_TYPE(Type_2, Obj_0, Obj_1, Obj_2, Obj_3) ||                    \
     ALL_OBJS_ARE_OF_TYPE(Type_3, Obj_0, Obj_1, Obj_2, Obj_3))

---

And FINALLY, putting it all together:

#define TEST_FUNC(a,b,c,d)                                              \
do                                                                      \
{                                                                       \
    STATIC_ASSERT(IS_ACCEPTABLE(uint8_t, uint16_t, uint32_t, uint64_t,  \
                                a,       b,        c,        d));       \
} while(0)

---

Of course, you could separate the above into more distinct, individual STATIC_ASSERTs, as you wish, if you want more verbose error output if any of the _Static_asserts fail.