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.
SUSv2, POSIX.1-2001. POSIX.1-2008 removes the
specification of getcontext(),
citing portability issues, and recommending that applications
be rewritten to use POSIX threads instead.
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.
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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Copyright (C) 2001 Andries Brouwer (aebcwi.nl) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. |