I would like to understand how overriding a set C struct is possible?

Question

In building a module for Apache web server, I have encountered several instances where the declaration of a structure says that it only takes const char* members as in the declaration of apr_table_t and apr_array_header_t, but examples I am finding in various modules such as mod_security, and even the new ap_parse_form_data function indicate that a void * data type is being inserted into these structures.

My questions are how is this possible and why will my Visual Studio compiler complain if I attempt to use the same method?

A good example of this is mod_security with a create_request function that stores a void * in a request_rec note.

Answer 1

Pointers are convertible, and every object pointer type is convertible to void*, in the sense that storing any pointer in a void* doesn't lose information. That is to say, the following is valid:

struct Foo a;
struct Foo * p = &a;   // p points to a

void * q = p;          // OK, can store the value of p in q

struct Foo * r = q;    // OK too - we can go back, and r points to a

r->n = 1;

Thus it is customary in C to pass around pointers as void*, knowing that those can always hold any other object pointer value, and only cast it back to the desired type when needed.

One final word about char pointers in particular: Casting any object pointer to char* and actually dereferencing it as a pointer to an array of chars does not constitute type punning and is not a violation of the strict aliasing rule -- this simply allows you to access the underlying binary representation of any object.