int getcontext(ucontext_t *ucp);
int setcontext(const ucontext_t *ucp);
The mcontext_t type is machine-dependent and opaque. The ucontext_t type is a structure that has at least the following fields:
typedef struct ucontext {
struct ucontext *uc_link;
sigset_t uc_sigmask;
stack_t uc_stack;
mcontext_t uc_mcontext;
...
} ucontext_t;
with sigset_t and stack_t defined in
<signal.h>.
Here uc_link points to the context that will be resumed
when the current context terminates (in case the current context
was created using
makecontext(3)),
uc_sigmask is the
set of signals blocked in this context (see
sigprocmask(2)),
uc_stack is the stack used by this context (see
sigaltstack(2)),
and uc_mcontext is the
machine-specific representation of the saved context,
that includes the calling thread's machine registers.
The function getcontext() initializes the structure pointed at by ucp to the currently active context.
The function setcontext() restores the user context pointed at by ucp. A successful call does not return. The context should have been obtained by a call of getcontext(), or makecontext(3), or passed as third argument to a signal handler.
If the context was obtained by a call of getcontext(), program execution continues as if this call just returned.
If the context was obtained by a call of makecontext(3), program execution continues by a call to the function func specified as the second argument of that call to makecontext(3). When the function func returns, we continue with the uc_link member of the structure ucp specified as the first argument of that call to makecontext(3). When this member is NULL, the thread exits.
If the context was obtained by a call to a signal handler, then old standard text says that "program execution continues with the program instruction following the instruction interrupted by the signal". However, this sentence was removed in SUSv2, and the present verdict is "the result is unspecified".
When a signal occurs, the current user context is saved and a new context is created by the kernel for the signal handler. Do not leave the handler using longjmp(3): it is undefined what would happen with contexts. Use siglongjmp(3) or setcontext() instead.