Hcreate(3)

https://man.archlinux.org/

Useful if you need to see the manpage of a very recent version of a package.

Reentrancy was an issue on consumer devices well before multithreading was commonly supported there, after all

(it's also true, as kimixa points out, that a single-threaded process with signal handlers isn't really single-threaded, but i don't think that's really the important consideration in this context)

for what it's worth, it's not very difficult to use alarm() to invoke a context-switching subroutine to get preemptive user-space multithreading on unix without any kernel support. in theory you have to write the context-switching subroutine in assembly language, which isn't difficult at all; here's one i wrote in arm assembly for the arm eabi, for cooperative multithreading:

            .thumb_func
            .globl einyield
    einyield:
            push {r0, r4-r11, lr}   @ save all callee-saved regs plus r0
            ldr r0, =current_task_pointer
            ldr r1, [r0]
            str sp, [r1]            @ save stack pointer in current eintask
            ldr r1, [r1, #4]        @ load pointer to next eintask
            str r1, [r0]
            ldr sp, [r1]            @ switch to next eintask's stack
            pop {r0, r4-r11, pc}    @ return into einyielded context there

but in practice you can usually just use longjmp()

preemptive multithreading requires you to save all the registers, not just the callee-saved registers, but so does signal handling, so you can usually just use the signal-handling machinery and use the above code (or its equivalent on your vax or 68000 or whatever) to context-switch between signal-handler stack frames