man3/getcontext.3

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.TH GETCONTEXT 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
getcontext, setcontext \- get or set the user context
.SH SYNOPSIS
.B #include <ucontext.h>
.PP
.BI "int getcontext(ucontext_t *" ucp );
.br
.BI "int setcontext(const ucontext_t *" ucp );
.SH DESCRIPTION
In a System V-like environment, one has the two types
.I mcontext_t
and
.I ucontext_t
defined in
.I <ucontext.h>
and the four functions
.BR getcontext (),
.BR setcontext (),
.BR makecontext (3),
and
.BR swapcontext (3)
that allow user-level context switching between multiple
threads of control within a process.
.PP
The
.I mcontext_t
type is machine-dependent and opaque.
The
.I ucontext_t
type is a structure that has at least
the following fields:
.PP
.in +4
.EX
typedef struct ucontext_t {
    struct ucontext_t *uc_link;
    sigset_t          uc_sigmask;
    stack_t           uc_stack;
    mcontext_t        uc_mcontext;
    ...
} ucontext_t;
.EE
.in
.PP
with
.IR sigset_t
and
.I stack_t
defined in
.IR <signal.h> .
Here
.I uc_link
points to the context that will be resumed
when the current context terminates (in case the current context
was created using
.BR makecontext (3)),
.I uc_sigmask
is the
set of signals blocked in this context (see
.BR sigprocmask (2)),
.I uc_stack
is the stack used by this context (see
.BR sigaltstack (2)),
and
.I uc_mcontext
is the
machine-specific representation of the saved context,
that includes the calling thread's machine registers.
.PP
The function
.BR getcontext ()
initializes the structure
pointed at by
.I ucp
to the currently active context.
.PP
The function
.BR setcontext ()
restores the user context
pointed at by
.IR ucp .
A successful call does not return.
The context should have been obtained by a call of
.BR getcontext (),
or
.BR makecontext (3),
or passed as third argument to a signal
handler.
.PP
If the context was obtained by a call of
.BR getcontext (),
program execution continues as if this call just returned.
.PP
If the context was obtained by a call of
.BR makecontext (3),
program execution continues by a call to the function
.I func
specified as the second argument of that call to
.BR makecontext (3).
When the function
.I func
returns, we continue with the
.I uc_link
member of the structure
.I ucp
specified as the
first argument of that call to
.BR makecontext (3).
When this member is NULL, the thread exits.
.PP
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".
.SH RETURN VALUE
When successful,
.BR getcontext ()
returns 0 and
.BR setcontext ()
does not return.
On error, both return \-1 and set
.I errno
appropriately.
.SH ERRORS
None defined.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw26 lb lb
l l l.
Interface	Attribute	Value
T{
.BR getcontext (),
.BR setcontext ()
T}	Thread safety	MT-Safe race:ucp
.TE
.SH CONFORMING TO
SUSv2, POSIX.1-2001.
POSIX.1-2008 removes the specification of
.BR getcontext (),
citing portability issues, and
recommending that applications be rewritten to use POSIX threads instead.
.SH NOTES
The earliest incarnation of this mechanism was the
.BR setjmp (3)/ longjmp (3)
mechanism.
Since that does not define
the handling of the signal context, the next stage was the
.BR 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
.BR getcontext ()
is from the first call, or via a
.BR setcontext ()
call.
The user has to invent her own bookkeeping device, and a register
variable won't do since registers are restored.
.PP
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
.BR longjmp (3):
it is undefined what would happen with contexts.
Use
.BR siglongjmp (3)
or
.BR setcontext ()
instead.
.SH SEE ALSO
.BR sigaction (2),
.BR sigaltstack (2),
.BR sigprocmask (2),
.BR longjmp (3),
.BR makecontext (3),
.BR sigsetjmp (3)