How Can I Convert a Void Pointer to a Function Pointer in C ?

Casting Function Pointers in C

Type conversion of void pointers to function pointers is a common scenario in programming. However, in C , converting a void to a function pointer directly is not allowed, as defined by the C 98/03 standard. This is because a void, according to the standard, is intended to point to objects, not function pointers or member pointers.

Despite the restricted casting in the C 98/03 standard, there are ways to achieve the conversion in certain contexts. These methods are implementation-dependent and may behave differently based on the compiler and operating system.

Option 1: Double Reinterpretation Cast

One approach is to use a double reinterpretation cast:

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

Here, the gptr is first cast to a long integer, and then the cast is performed from the long to the target function pointer type. This approach is not guaranteed to work across all platforms and is not part of the C standard.

Option 2: Void and Address Manipulation**

An alternative method involves manipulating void** and addresses:

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

In this case, a void is utilized to store the address of the function pointer. By assigning the address from gptr to the void, the function pointer's address is accessible indirectly. This method is also implementation-dependent.

It's crucial to note that these techniques exploit specific platform behaviors and are not well-defined by the C standard. While they may function in most cases, it's important to be aware of potential inconsistencies and portability issues.

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