getcontext, setcontext — get or set the user context
#include <ucontext.h>
int
getcontext( |
ucontext_t * | ucp) ; |
int
setcontext( |
const ucontext_t * | ucp) ; |
In a System V-like environment, one has the two types
mcontext_t
and
ucontext_t
defined
in <ucontext.h>
and the
four functions getcontext
(),
setcontext
(), makecontext(3) and swapcontext(3) that allow
user-level context switching between multiple threads of
control within a process.
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 successful, getcontext
() returns 0 and setcontext
() does not return. On error,
both return −1 and set errno
appropriately.
The earliest incarnation of this mechanism was the
setjmp
(3)/longjmp
(3) mechanism. Since that does not
define the handling of the signal context, the next stage was
the sigsetjmp
(3)/siglongjmp
(3) pair. The present mechanism
gives much more control. On the other hand, there is no easy
way to detect whether a return from getcontext
() is from the first call, or via
a setcontext
() call. The user
has to invent her own bookkeeping device, and a register
variable won't do since registers are restored.
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.
sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3), makecontext(3), sigsetjmp(3)
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