Can C Convert a Void Pointer to a Function Pointer?

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:

<code class="cpp">void *gptr = dlsym(some symbol..);
typedef void (*fptr)();
fptr my_fptr = reinterpret_cast<fptr>(reinterpret_cast<long>(gptr));</code>

Alternatively:

<code class="cpp">fptr my_ptr = 0;
reinterpret_cast<void*&>(my_ptr) = gptr;</code>

Complex but Portable Approach:

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

<code class="cpp">// Get address as object pointer
void (**object_ptr)() = &my_ptr;

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

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

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

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