Segfault when calling GetBinaryTypeA - winapi

Question

I tried to import the GetBinaryTypeA function:
use std::ffi::CString;
use ::std::os::raw::{c_char, c_ulong};
extern { fn GetBinaryTypeA(s: *const c_char, out: *mut c_ulong) -> i32; }
fn main() {
let path = "absolute/path/to/bin.exe";
let cpath = CString::new(path).unwrap();
let mut out: c_ulong = 0;
println!("{:?}", cpath);
unsafe { GetBinaryTypeA(cpath.as_ptr(), out as *mut c_ulong); }
println!("{:?}", cpath);
}
Output:
error: process didn't exit successfully: `target\debug\bin_deploy.exe` (exit code: 3221225477)
Process finished with exit code -1073741819 (0xC0000005)
If I set an invalid path then it executes successfully and GetLastError() returns 2 ("The system cannot find the file specified"), so it looks like the imported function works.
I received the same error using the kernel32-sys crate. Where else can the error be?

Answer 1

You are casting the value 0 to a pointer. On the vast majority of computers in use today, the pointer with the value 0 is known as NULL. Thus, you are trying to write to the NULL pointer, which causes a crash.
You want to write to the address of the value:
&mut out as *mut c_ulong
Which doesn't even need the cast:
unsafe {
GetBinaryTypeA(cpath.as_ptr(), &mut out);
}