makecontext, swapcontext - set up user context

Syntax	#include <ucontext.h> void makecontext (ucontext_t *ucp, (void *func) (), int argc,...); int swapcontext (ucontext_t *oucp, const ucontext_t *ucp);
Description	These functions serve to implement a change of context between several control flows within a user process. makecontext() changes the context specified by ucp which was initialized via getcontext(). If this context is activated with swapcontext() or setcontext() (see getcontext()), the program execution is continued with the call of the function func. The arguments which follow argc are passed to makecontext(). The integer value of argc must correspond to the number of arguments that follow argc. Otherwise, the behavior is undefined. Before makecontext() is called, the context to be modified should be assigned a stack. The structure element uc_link defines the context that is activated when the context modified by makecontext() returns. swapcontext() saves the current context in the context structure pointed to by oucp, and sets the context to the context structure pointed to by ucp.
Return val.	0 -1	after successful execution of swapcontext(). if an error occurs. errno is set to indicate the type of the error.
Errors	These functions will fail if: ENOMEM      ucp no longer has enough space in the stack to perform the operation.
See also	exit(), getcontext(), sigaction(), sigprocmask(), ucontext.h.