Misusing bbThreadSetData() and bbThreadGetData() in BlitzMax NG

test.bmx

' This code works on x64 windows, but fails on arm64
' That occurs because the upper 4 bytes of the pointer get cut off and arm relies on them
' I.e. 0x0000007f88799fc0 turns into 0xffffffff88799fc0 on arm
' On x64 it doesn't crash because the upper 4 bytes are just so happen to be unused
' I.e. 0000000003139FC0 still remains 0000000003139FC0
' You _can_ make it work on arm if you manually replace the higher 0xffffffff back to 0x0000007f
'
' This code is only for documenting purposes

Framework BRL.StandardIO
Import BRL.Threads

Import "thread_hack.c"

Extern 
	Function set_thread_data(data:SomeData)
	Function get_thread_data:SomeData()
End Extern

Type SomeData
	Field A:String
	Field B:String
End Type

Local Inst:SomeData = New SomeData

Inst.A = "Hello"
Inst.B = "World"

set_thread_data(Inst)

Print "Data has been set. Will now attempt to recover it."

Print get_thread_data().A + get_thread_data().B

thread_hack.c

void set_thread_data(struct _m_test_SomeData_obj* data) {
	printf("Got a pointer: %p\n", data);
	
	bbThreadSetData(16, data);
}

struct _m_test_SomeData_obj* get_thread_data() {
	struct _m_test_SomeData_obj* data;
	
	data = (struct _m_test_SomeData_obj*)bbThreadGetData(16);
	
	printf("Recovered a pointer: %p\n", data);
	
	return data;
}

Forgot to mention, the entire point of this code is that data set by set_thread_data() and recovered by get_thread_data() will be unique to the caller thread. It's a hacky example on how you can implement Thread Local Storage, that's what it is.