Originální popis anglicky:
getcontext, setcontext - get or set the user context
Návod, kniha: Linux Programmer's Manual
#include <ucontext.h>
int getcontext(ucontext_t *ucp);
int setcontext(const ucontext_t *ucp);
In a SysV-like environment, one has the two types
mcontext_t and
ucontext_t defined in
<ucontext.h> and the four functions
getcontext(),
setcontext(),
makecontext() and
swapcontext() 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()),
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(), 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(), program execution
continues by a call to the function
func specified as the second
argument of that call to
makecontext(). 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(). 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.
None defined.
The earliest incarnation of this mechanism was the
setjmp()/
longjmp() mechanism. Since that does not define the
handling of the signal context, the next stage was the
sigsetjmp()/
siglongjmp() 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() - it is undefined what would happen with contexts. Use
siglongjmp() or
setcontext() instead.
SUSv2
sigaction(2),
sigaltstack(2),
sigprocmask(2),
longjmp(3),
makecontext(3),
sigsetjmp(3)
