getcontext

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



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 their
       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 5.03 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/.



Linux                             2017-09-15                     GETCONTEXT(3)