GETCONTEXT
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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NAME
getcontext, setcontext - get or set the user context
SYNOPSIS
#include <ucontext.h>
int getcontext(ucontext_t *ucp);
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
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_t {
struct ucontext_t *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".
RETURN VALUE
When successful,
getcontext()
returns 0 and
setcontext()
does not return.
On error, both return -1 and set
errno
appropriately.
ERRORS
None defined.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
getcontext(),
setcontext()
| Thread safety | MT-Safe race:ucp
|
CONFORMING TO
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.
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
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.
SEE ALSO
sigaction(2),
sigaltstack(2),
sigprocmask(2),
longjmp(3),
makecontext(3),
sigsetjmp(3)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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- Running on 
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