Multitasking using setjmp, longjmp
It may be bending the rules a little, but GNU pth does this. It's possible, but you probably shouldn't try it yourself except as an academic proof-of-concept exercise, use the pth implementation if you want to do it seriously and in a remotely portable fashion -- you'll understand why when you read the pth thread creation code.
(Essentially it uses a signal handler to trick the OS into creating a fresh stack, then longjmp's out of there and keeps the stack around. It works, evidently, but it's sketchy as hell.)
In production code, if your OS supports makecontext/swapcontext, use those instead. If it supports CreateFiber/SwitchToFiber, use those instead. And be aware of the disappointing truth that one of the most compelling use of coroutines -- that is, inverting control by yielding out of event handlers called by foreign code -- is unsafe because the calling module has to be reentrant, and you generally can't prove that. This is why fibers still aren't supported in .NET...
You can indeed. There are a couple of ways to accomplish it. The difficult part is initially getting the jmpbufs which point to other stacks. Longjmp is only defined for jmpbuf arguments which were created by setjmp, so there's no way to do this without either using assembly or exploiting undefined behavior. User level threads are inherently not portable, so portability isn't a strong argument for not doing it really.
step 1 You need a place to store the contexts of different threads, so make a queue of jmpbuf stuctures for however many threads you want.
Step 2 You need to malloc a stack for each of these threads.
Step 3 You need to get some jmpbuf contexts which have stack pointers in the memory locations you just allocated. You could inspect the jmpbuf structure on your machine, find out where it stores the stack pointer. Call setjmp and then modify its contents so that the stack pointer is in one of your allocated stacks. Stacks usually grow down, so you probably want your stack pointer somewhere near the highest memory location. If you write a basic C program and use a debugger to disassemble it, and then find instructions it executes when you return from a function, you can find out what the offset ought to be. For example, with system V calling conventions on x86, you'll see that it pops %ebp (the frame pointer) and then calls ret which pops the return address off the stack. So on entry into a function, it pushes the return address and frame pointer. Each push moves the stack pointer down by 4 bytes, so you want the stack pointer to start at the high address of the allocated region, -8 bytes (as if you just called a function to get there). We will fill the 8 bytes next.
The other thing you can do is write some very small (one line) inline assembly to manipulate the stack pointer, and then call setjmp. This is actually more portable, because in many systems the pointers in a jmpbuf are mangled for security, so you can't easily modify them.
I haven't tried it, but you might be able to avoid the asm by just deliberately overflowing the stack by declaring a very large array and thus moving the stack pointer.
Step 4 You need exiting threads to return the system to some safe state. If you don't do this, and one of the threads returns, it will take the address right above your allocated stack as a return address and jump to some garbage location and likely segfault. So first you need a safe place to return to. Get this by calling setjmp in the main thread and storing the jmpbuf in a globally accessible location. Define a function which takes no arguments and just calls longjmp with the saved global jmpbuf. Get the address of that function and copy it to your allocated stacks where you left room for the return address. You can leave the frame pointer empty. Now, when a thread returns, it will go to that function which calls longjmp, and jump right back into the main thread where you called setjmp, every time.
Step 5 Right after the main thread's setjmp, you want to have some code that determines which thread to jump to next, pulling the appropriate jmpbuf off the queue and calling longjmp to go there. When there are no threads left in that queue, the program is done.
Step 6 Write a context switch function which calls setjmp and stores the current state back on the queue, and then longjmp on another jmpbuf from the queue.
Conclusion That's the basics. As long as threads keep calling context switch, the queue keeps getting repopulated, and different threads run. When a thread returns, if there are any left to run, one is chosen by the main thread, and if none are left, the process terminates. With relatively little code you can have a pretty basic cooperative multitasking setup. There are more things you probably want to do, like implement a cleanup function to free the stack of a dead thread, etc. You can also implement preemption using signals, but that is much more difficult because setjmp doesn't save the floating point register state or the flags registers, which are necessary when the program is interrupted asynchronously.
This is a form of what is known as userspace context switching.
It's possible but error-prone, especially if you use the default implementation of setjmp and longjmp. One problem with these functions is that in many operating systems they'll only save a subset of 64-bit registers, rather than the entire context. This is often not enough, e.g. when dealing with system libraries (my experience here is with a custom implementation for amd64/windows, which worked pretty stable all things considered).
That said, if you're not trying to work with complex external codebases or event handlers, and you know what you're doing, and (especially) if you write your own version in assembler that saves more of the current context (if you're using 32-bit windows or linux this might not be necessary, if you use some versions of BSD I imagine it almost definitely is), and you debug it paying careful attention to the disassembly output, then you may be able to achieve what you want.
I did something like this for studies. https://github.com/Kraego/STM32L476_MiniOS/blob/main/Usercode/Concurrency/scheduler.c
The context/thread switching is done by setjmp/longjmp. The difficult part was to get the allocated stack correct (see allocateStack()) this depends on your platform.
This is just a demonstration how this could work, I would never use this in production.