getcontext(2) — Linux manual page


GETCONTEXT(3)             Linux Programmer's Manual            GETCONTEXT(3)

NAME         top

       getcontext, setcontext - get or set the user context

SYNOPSIS         top

       #include <ucontext.h>

       int getcontext(ucontext_t *ucp);
       int setcontext(const ucontext_t *ucp);

DESCRIPTION         top

       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 termi‐
       nates (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 representa‐
       tion of the saved context, that includes the calling thread's machine

       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 execu‐
       tion continues as if this call just returned.

       If the context was obtained by a call of makecontext(3), program exe‐
       cution continues by a call to the function func specified as the sec‐
       ond 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         top

       When successful, getcontext() returns 0 and setcontext() does not
       return.  On error, both return -1 and set errno appropriately.

ERRORS         top

       None defined.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                  Attribute     Value            │
       │getcontext(), setcontext() │ Thread safety │ MT-Safe race:ucp │

CONFORMING TO         top

       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         top

       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 their own bookkeeping
       device, and a register variable won't do since registers are

       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         top

       sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3),
       makecontext(3), sigsetjmp(3)

COLOPHON         top

       This page is part of release 5.07 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

Linux                            2017-09-15                    GETCONTEXT(3)