How to check if function pointer passed from C is non-NULL

Question

Example code below

The Rust part:

#[no_mangle]
pub extern fn call_c_function(value: i32, fun: fn(i32) -> i32) -> i32 {
    fun(value)
}
         


        

Answer 1

You can use Option<...> to represent nullable function pointers. It is incorrect to have a NULL value for a value of type fn(...) so the Option wrapper is required for cases like this.

For example,

#[no_mangle]
pub extern "C" fn call_c_function(value: i32, fun: Option<fn(i32) -> i32>) -> i32 {
    if let Some(f) = fun {
        f(value)
    }
}

However, there's one extra point: fun is a C function, but the type fn(...) is a Rust function. They're not directly compatible (e.g. their calling conventions differ). One needs to use the extern "C" fn(...) (a.k.a. extern fn(...)) type when interacting with C function pointers:

#[no_mangle]
pub extern "C" fn call_c_function(value: i32, fun: Option<extern "C" fn(i32) -> i32>) -> i32 {
    if let Some(f) = fun {
        f(value)
    }
}

Answer 2

You can compare a pointer that has been generated by unsafe code against std::ptr::null()

e.g.

let pw = libc::getpwnam(username.as_ptr() as *const i8);
if std::ptr::null() != pw ...

null() on Linux is 0 as *const T I'm not sure if that is universal.