I've not done much rust so i've jumped off at the deep end to by doing some FFI C calls. I'm unsure of the correct way to make one of my calls.

I have a C function which has the signature something like
void f(char** buf);
In C you can do these two calls.

char* buf;
f(&buf); // Function returns a pointer to something and does something
f(NULL); // Function does something but doesn't return a pointer.

1. What would be the correct signature for this function in rust? I hope I don't embarrass my self but I have this. fn f(buf : &*mut libc::c_void);?
2. How should I do the two different calls?

let mut buf : [i8; 5] = [1,2,3,4,5];
unsafe{f(& buf.as_mut_ptr());}  // seems to work but I'm not entirely sure it is correct.

unsafe{f(& *ptr::null_mut());}   // I get the warning 'this code causes undefined behavior when executed'

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