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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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.