The makecontext function modifies the user thread context pointed to by ucp, which must have previously been initialized by a call to getcontext(3) and had a stack allocated for it. The context is modified so that it will continue execution by invoking func with the arguments provided. The argc argument must be equal to the number of additional arguments provided to makecontext and also equal to the number of arguments to func, or else the behavior is undefined.
The "ucp->uc_link" argument must be initialized before calling makecontext and determines the action to take when func returns: if equal to NULL, the process exits; otherwise, setcontext "ucp->uc_link" is implicitly invoked.
The swapcontext function saves the current thread context in "*oucp" and makes "*ucp" the currently active context.
If successful, swapcontext returns zero; otherwise -1 is returned and the global variable errno is set appropriately.
The swapcontext function will fail if: