gcc - How to get the type of a variable in C code?

Question

Is there any way that I can discover the type of a variable automatically in C, either through some mechanism within the program itself, or--more likely--through a pre-compilation script that uses the compiler's passes up to the point where it has parsed the variables and assigned them their types? I'm looking for general suggestions about this. Below is more background about what I need and why.

I would like to change the semantics of the OpenMP reduction clause. At this point, it seems easiest simply to replace the clause in the source code (through a script) with a call to a function, and then I can define the function to implement the reduction semantics I want. For instance, my script would convert this

#pragma omp parallel for reduction(+:x)

into this:

my_reduction(PLUS, &x, sizeof(x));
#pragma omp parallel for

where, earlier, I have (say)

enum reduction_op {PLUS, MINUS, TIMES, AND,
  OR, BIT_AND, BIT_OR, BIT_XOR, /* ... */};

And my_reduction has signature

void my_reduction(enum reduction_op op, void * var, size_t size);

Among other things, my_reduction would have to apply the addition operation to the reduction variable as the programmer had originally intended. But my function cannot know how to do this correctly. In particular, although it knows the kind of operation (PLUS), the location of the original variable (var), and the size of the variable's type, it does not know the variable's type itself. In particular, it does not know whether var has an integral or floating-point type. From a low-level POV, the addition operation for those two classes of types is completely different.

If only the nonstandard operator typeof, which GCC supports, would work the way sizeof works--returning some sort of type variable--I could solve this problem easily. But typeof is not really like sizeof: it can only be used, apparently, in l-value declarations.

Now, the compiler obviously does know the type of x before it finishes generating the executable code. This leads me to wonder whether I can somehow leverage GCC's parser, just to get x's type and pass it to my script, and then run GCC again, all the way, to compile my altered source code. It would then be simple enough to declare

enum var_type { INT8, UINT8, INT16, UINT16, /* ,..., */ FLOAT, DOUBLE};
void my_reduction(enum reduction_op op, void * var, enum var_type vtype);

And my_reduction can cast appropriately before dereferencing and applying the operator.

As you can see, I am trying to create a kind of "dispatching" mechanism in C. Why not just use C++ overloading? Because my project constrains me to work with legacy source code written in C. I can alter the code automatically with a script, but I cannot rewrite it into a different language.

Thanks!

Answer 1

C11 _Generic

Not a direct solution, but it does allow you to achieve the desired result if you are patient to code all types as in:

#include <assert.h>
#include <string.h>

#define typename(x) _Generic((x), 
    int:     "int", 
    float:   "float", 
    default: "other")

int main(void) {
    int i;
    float f;
    void* v;
    assert(strcmp(typename(i), "int")   == 0);
    assert(strcmp(typename(f), "float") == 0);
    assert(strcmp(typename(v), "other") == 0);
}

Compile and run with:

gcc -std=c11 a.c
./a.out

A good starting point with tons of types can be found in this answer.

Tested in Ubuntu 17.10, GCC 7.2.0. GCC only added support in 4.9.