void makecontext (ucontext_t ucp, (void func)(), int argc, . . . );
int swapcontext (ucontext_t oucp, const ucontext_t ucp);
These functions are useful for implementing user-level context switching between multiple threads of control within a process.
makecontext modifies the context specified by ucp, which has been initialized using getcontext; when this context is resumed using swapcontext or setcontext [see getcontext(S)], program execution continues by calling the function func, passing it the arguments that follow argc in the makecontext call. Before a call is made to makecontext, the context being modified should have a stack allocated for it. The value of argc must match the number of integers passed to func, otherwise the behavior is undefined.
uc_link field is used to determine the context that will be
resumed when the context being modified by makecontext returns.
uc_link field should be initialized prior to the call to
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.
Equivalent routines and an enhanced user context structure are also provided in libc to handling floating point context changes for Pentium III processors. See ``Pentium III extended floating point support'' in New features for more information.