GETCONTEXT
NAME
getcontext, setcontext - get or set the user context
SYNOPSIS
#include <ucontext.h>
I int getcontext(ucontext_t * ucp );
I int setcontext(const ucontext_t * ucp );
DESCRIPTION
In a System V-like environment, one has the two types
mcontext_t and
ucontext_t defined in
<ucontext.h>
and the four functions
R getcontext (),
R 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
R <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
R getcontext ()
initializes the structure
pointed at by ucp to the currently active context.
The function
R 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
R getcontext (),
or
makecontext(3),
or passed as third argument to a signal
handler.
If the context was obtained by a call of
R 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".
RETURN VALUE
When successful,
R getcontext ()
returns 0 and
R setcontext ()
does not return.
On error, both return -1 and set errno
appropriately.
ERRORS
None defined.
CONFORMING TO
SUSv2, POSIX.1-2001.
NOTES
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
R getcontext ()
is from the first call, or via a
R 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
R setcontext ()
instead.
SEE ALSO
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