Converting Void Pointers to Function Pointers: A Conditional Possibility in C

Problem

Obtaining a void pointer via dlsym(), the goal is to invoke the function referenced by that pointer. The attempted conversion through casting using static_cast or reinterpret_cast has failed, unlike a C-style cast.

Answer

Direct conversion of a void pointer to a function pointer is impermissible in C 98/03. However, C 0x offers conditional support, allowing an implementation to dictate the behavior.

Undefined Behavior Approach:

While undefined by the standard, the following code may work on most platforms:

void *gptr = dlsym(some symbol..);
typedef void (*fptr)();
fptr my_fptr = reinterpret_cast(reinterpret_cast(gptr));

Alternatively:

fptr my_ptr = 0;
reinterpret_cast(my_ptr) = gptr;

Complex but Portable Approach:

This method exploits the fact that a function pointer's address is an object pointer:

// Get address as object pointer
void (**object_ptr)() = &my_ptr;

// Convert to void** (also an object pointer)
void **ppv = reinterpret_cast(object_ptr);

// Store address from 'gptr' in memory cell pointed to by 'ppv'
*ppv = gptr;

This approach remains undefined in the standard but should function reasonably well on most implementations.