This is an article I wish I could have read many months ago.

> Hence, the most basic safety issue with setjmp is that if we call it and then return from the function that had called it, the context saved by setjmp is not valid to longjmp to.

> longjmp is only safe if it's called at a time when the stack frame used by setjmp could not have possibly been overwritten, since that is the only way to guarantee that the register state restored by longjmp matches the stack frame that the stack pointer points to.

That limitation could be lifted by simply copying the stack frames somewhere else prior to long jumping, and then spilling that entire thing on top of the current stack instead of just restoring the registers from the jump buffer. This is how delimited continuations work! What ruins this for C is the existence of pointers. Stacks aren't freely relocatable since pointers into the stack could exist. Other languages don't have this problem.

So much fun stuff in this article! The "fibers with ucontext", essentially swapping stack pointers back and forth, are how I implemented generators! I too reached for musl source code in order to understand setjmp, but for a different reason: its ability to spill the registers onto the stack was instrumental for my garbage collector.

Blogged about all of these things too, in case anyone is curious:

https://www.matheusmoreira.com/articles/delimited-continuati...

https://www.matheusmoreira.com/articles/generators-in-lone-l...

https://www.matheusmoreira.com/articles/babys-second-garbage...