NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON
GETCONTEXT(2) Linux Programmer's Manual GETCONTEXT(2)
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.
None defined.
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.
sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3), makecontext(3),
sigsetjmp(3)
This page is part of release 3.23 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/.
Linux 2009-03-15 GETCONTEXT(2)