How are greenlets implemented? Python uses the C stack for the interpreter and it heap-allocates Python stack frames, but beyond that, how does it allocate/swap stacks, how does it hook into the interpreter and function call mechanisms, and how does this interact with C extensions? (Any quirks)?
There are some comments at the top of greenlet.c in the source, but they're a bit opaque.
(One data point: they don't use ucontext.h.)
If get and study the greenlet's sources, you'll see at the top of greenlet.c a long comment that starts at line 16 with the following summary...:
A PyGreenlet is a range of C stack addresses that must be saved and restored in such a way that the full range of the stack contains valid data when we switch to it.
and continues to line 82, summarizing exactly what you're asking about. Have you studies these lines (and the following 1000+ implementing them;-)...? I don't see a way to further squeeze these 66 lines down while still making sense, nor any added value in copying and pasting them here.
Basically, you'll see there is no real "hooking" to speak of (the C level stack is switched back and forth "under the interpreter's nose", so to speak) except for the delicate interactions with thread state in multi-threaded code, and the saving and restoring of a greenlet's state from/to the stack is based on memcpy calls plus some calls to the Python memory manager to allocate/reallocate and free space coming from, or going back to, the stack. The three functions in line 227-295 handle the grunt work, and they're wrapped in a couple C macros at 298-310 "in order to simplify maintenance", as the comment there says.
The interface through which other C extensions can interact with the greenlet extension is implemented at lines 956-1045, and exposed through the "CObject API" (via greenlet.h, of course) documented here.