On 10/25/20 3:19 AM, rikki cattermole wrote:
On 25/10/2020 11:03 PM, Ali Çehreli wrote:
Does the GC see that local variable 'name' that is on the C side? What
I don't know is whether the GC is aware only of the stack frames of D
functions or the entire thread, which would include the C caller's
'name'.
The thread stack frame that is registered with the D GC will know about
the D side and may know about the C side.
It depends on what the C side is doing.
If the C side went ahead and made a new stack frame via a fiber... it
won't know about it. But even if it did, the D stack frame is still
alive and pinning that bit of memory.
Ultimately, if the C side puts that pointer some place like a global or
send it to another thread, there are no guarantees that things will play
out well.
Sorry to bring this up again but I want to understand this fully before
I say something wrong during my DConf presentation. :)
The D code is a library. The actual program is e.g. written in C. When
the D library is loaded into the program, the following function is
executed and the D GC is initialized:
pragma (crt_constructor)
extern(C) int initialize() {
return rt_init();
}
Does the D GC know the complete function call stack of the C program all
the way up from 'main'? Is there the concept of "bottom of the stack" or
does the D GC can only know the value of the stack pointer at the time
rt_init() was called. If the latter, then I think a toStringz string may
not be alive in a C function.
Imagine the C program dlopens our library from inside a function called
from main. Then the program calls one of our library functions from
another function in main:
// C program
int main() {
initializeDlibrary(); // This does dlopen()
useDlibrary(); // This receives a string returned from
// toStringZ and uses that string.
}
So, the question is, does D GC only know initializeDlibrary's stack
frame up because it was initialized there?
I know threads complicate matters and they need to be attached to the GC
with core.thread.osthread.thread_attachThis but I am not there yet. :) I
want to understand the basic single thread stack pointer issue first.
Thank you,
Ali
