How Can I Safely Cast a `void*` to a Function Pointer in C ?

Function Pointer Casting in C

In C , converting a void* to a function pointer directly is generally not allowed. However, there are a few approaches that may work depending on the implementation and platform.

Method 1 (Undefined Behavior):

This method involves double reinterpret_casting:

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

Method 2 (Avoids Direct Conversion):

This method uses an intermediate step to store the function pointer address in a variable:

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

Slower but More Transparent Version of Method 2:

<code class="cpp">void (**object_ptr)() = &my_ptr;

void ** ppv = reinterpret_cast<void**>(object_ptr);

*ppv = gptr;</code>

Limitations and Caveats:

• These methods are implementation-dependent and their behavior is not guaranteed by the C standard.
• They may cause undefined behavior on some platforms or with certain compilers.
• It's recommended to always check the validity of the pointer before using it as a function pointer.

Note:

In C 0x, the behavior of converting a void* to a function pointer is conditionally supported.

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