GetAllocatedString crashes with STATUS_ACCESS_VIOLATION.

cpp_working_example.cpp

#include <iostream>
#include <string>
#include <mfapi.h>
#include <mfidl.h>

void listup_webcams() {
	IMFAttributes *p_attr = NULL;
	HRESULT hr = MFCreateAttributes(&p_attr, 1);
	if(FAILED(hr)){
		std::cout 
			<< std::hex
			<< "Failed. (MFCreateAttributes 0x" << hr << ")" 
			<< std::dec
			<< std::endl;
		return;
	}

	hr = p_attr->SetGUID(
		MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE, 
        MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE_VIDCAP_GUID);
	if(FAILED(hr)){
		std::cout 
			<< std::hex
			<< "Failed. (SetGUID 0x" << hr << ")" 
			<< std::dec
			<< std::endl;
		return;
	}

	IMFActivate **pp_devices = NULL;
	UINT32 count;
	hr = MFEnumDeviceSources(p_attr, &pp_devices, &count);
	if(FAILED(hr)){
		std::cout 
			<< std::hex
			<< "Failed. (GetString 0x" << hr << ")" 
			<< std::dec
			<< std::endl;
		return;
	}
	if(count == 0) {
		std::cout
			<< "There's no webcam device available. "
			<< std::endl;
	}

	for(unsigned int i = 0; i < count; i++) {
		UINT32 length = 0;
		WCHAR *p_str = NULL;
		hr = pp_devices[i]->GetAllocatedString(
			MF_DEVSOURCE_ATTRIBUTE_FRIENDLY_NAME, 
			&p_str, &length);
		if(FAILED(hr)){
			std::cout 
				<< std::hex
				<< "Failed. (GetAllocatedString 0x" << hr << ")" 
				<< std::dec
				<< std::endl;
			return;
		}

		std::wcout << "Device Name: " << p_str << std::endl;
	}

	if(pp_devices != NULL) {
		for(unsigned int i = 0; i < count; i++) {
			pp_devices[i]->Release();
		}
		CoTaskMemFree(pp_devices);
	}
}

int main() {
	listup_webcams();

	return 0;
}

rust_crash_example.rs

use std::ptr;
use windows::{
    core::*, 
    Win32::Media::MediaFoundation::*, 
};

fn main() -> Result<()>  {
    unsafe {
        let mut imf_attr_opt: Option<IMFAttributes> = None;
        MFCreateAttributes(&mut imf_attr_opt, 1)?;
        let imf_attr = imf_attr_opt.unwrap();
        println!("Set GUID. ");
        imf_attr.SetGUID(&MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE, &MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE_VIDCAP_GUID)?;
        
        let mut devices_opt_ptr: *mut Option<IMFActivate> = ptr::null_mut();
        let mut num_of_devices: u32 = 0;
        println!("Acquire webcam devices list. ");
        MFEnumDeviceSources(&imf_attr, &mut devices_opt_ptr, &mut num_of_devices)?;
        
        if num_of_devices == 0 {
            println!("There's no webcam device available. ");
            return Ok(())
        }
        else {
            println!("{} device[s] found. ", num_of_devices);
        }
        
        let mut length: u32 = 0;
        let device_name: *mut PWSTR = ptr::null_mut();
        println!("Enumerate webcam devices. ");
        for i in 0..num_of_devices {
            let device_opt: *mut Option<IMFActivate> = devices_opt_ptr.offset(i as isize);
            let device: &IMFActivate = (*device_opt).as_ref().unwrap();
            if let Ok(_) = device.GetAllocatedString(
                &MF_DEVSOURCE_ATTRIBUTE_FRIENDLY_NAME, 
                device_name, 
                &mut length) {
                
                println!("length: {}", length);
                let device_name = (*device_name).to_string()?;
                println!("{}: {}", i, device_name);
            }
            else {
                println!("Failed to allocate string. ");
            }
        }
    }
    Ok(())
}