Calling cdecl Functions That Have Different Number of Arguments - c

Question

I have functions that I wish to call based on some input. Each function has different number of arguments. In other words,
if (strcmp(str, "funcA") == 0) funcA(a, b, c);
else if (strcmp(str, "funcB") == 0) funcB(d);
else if (strcmp(str, "funcC") == 0) funcC(f, g);
This is a bit bulky and hard to maintain. Ideally, these are variadic functions (e.g., printf-style) and can use varargs. But they are not. So exploiting the cdecl calling convention, I am stuffing the stack via a struct full of parameters. I'm wondering if there's a better way to do it. Note that this is strictly for in-house (e.g., simple tools, unit tests, etc.) and will not be used for any production code that might be subjected to malicious attacks.
Example:
#include <stdio.h>
typedef struct __params
{
unsigned char* a;
unsigned char* b;
unsigned char* c;
} params;
int funcA(int a, int b)
{
printf("a = %d, b = %d\n", a, b);
return a;
}
int funcB(int a, int b, const char* c)
{
printf("a = %d, b = %d, c = %s\n", a, b, c);
return b;
}
int funcC(int* a)
{
printf("a = %d\n", *a);
*a *= 2;
return 0;
}
typedef int (*f)(params);
int main(int argc, char**argv)
{
int val;
int tmp;
params myParams;
f myFuncA = (f)funcA;
f myFuncB = (f)funcB;
f myFuncC = (f)funcC;
myParams.a = (unsigned char*)100;
myParams.b = (unsigned char*)200;
val = myFuncA(myParams);
printf("val = %d\n", val);
myParams.c = (unsigned char*)"This is a test";
val = myFuncB(myParams);
printf("val = %d\n", val);
tmp = 300;
myParams.a = (unsigned char*)&tmp;
val = myFuncC(myParams);
printf("a = %d, val = %d\n", tmp, val);
return 0;
}
Output:
gcc -o func func.c
./func
a = 100, b = 200
val = 100
a = 100, b = 200, c = This is a test
val = 200
a = 300
a = 600, val = 0

Answer 1

Rather than trying to torture this out of corner cases in the language, I would do this in a much more straightforward way, by defining variadic wrapper functions. You can do this generically with macros:
#define WRAP_VARIADIC_2_ARG(FNAME, RETTYPE, ARGTYPE1, ARGTYPE2) \
(RETTYPE) wrapped_##FNAME(...) { \
va_list args; \
va_start(args, 2); \
(ARGTYPE1) arg1 = va_arg(args, (ARGTYPE1)); \
(ARGTYPE2) arg2 = va_arg(args, (ARGTYPE2)); \
va_end(args); \
return FNAME(arg1, arg2); \
}
And similar macros for other arg counts. Then, you call:
WRAP_VARIADIC_2_ARG(funcA, int, int, int)
WRAP_VARIADIC_3_ARG(funcB, int, int, int, const char*)
WRAP_VARIADIC_1_ARG(funcC, int, int*)
This will define a set of functions with the following signatures that you can use in your dispatching function:
int wrapped_funcA(...)
int wrapped_funcB(...)
int wrapped_funcC(...)
This should be straightforward from there.