What is the difference between (void*) and (void(*)(argument type)) cast? [duplicate]

Question

void funcPtr(int a);

int main(){
   int k=1;
   void (*funcPtr2)(int);

   funcPtr2 = (void*)(funcPtr);
   // funcPtr2 = (void(*)(int))(funcPtr);

   (*funcPtr2)(k);
   return 0;
}

void funcPtr(int a){
   printf("%d", a);
}

What is the difference between (void*) and (void(*)(argument type) in function pointer type casting?

As a result, it does not occur warning.

Is this wrong? about (void*) type casting

Answer 1

Is this wrong? about (void*) type casting

Yes, it is.

C standard doesn't allow a conversion of a function pointer to an object pointer or assignment between them. If you up your compiler warning level, you may get warnings/errors such as compiling with:

gcc -Wall -Wextra -pedantic-errors -std=c11 file.c

I am not sure why you thought about casting a function pointer. Provided the function pointer type matches the function, simply assign it:

 funcPtr2 = funcPtr;

---

Aside:

You can use the function pointer just like a function:

 funcPtr2(k);

and use a standard prototype for main such as:

int main(void)

Answer 2

This is an issue with dlsym, declared void *dlsym(void *restrict handle, const char *restrict name). dlsym returns a pointer to a function named name defined in and accessible through handle.
However, as @BlueMoon states, this is not defined in ISO C and gcc indeed complains if made pedantic. POSIX tried to address that and required conforming implementation to make that cast well-defined in its indefiniteness.

From POSIX

Note that conversion from a void * pointer to a function pointer as in:

fptr = (int (*)(int))dlsym(handle, "my_function");

is not defined by the ISO C standard. This standard requires this conversion to work correctly on conforming implementations.

Another way to make that cast conforming is to first convert the function pointer to void** and then dereference it. This should be well-defined. In your case,

void funcPtr(int a); /* symbol to extract */

int main(void)
{
     void (*fn)(int);
     *(void**)(&fn) = dlsym(your_handle, "funcPtr");


     fn(5);
}