getcontext(3)                                         Library Functions Manual                                        getcontext(3)

       getcontext, setcontext - get or set the user context

       Standard C library (libc, -lc)

       #include <ucontext.h>

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

       In  a  System V-like environment, one has the two types mcontext_t and ucontext_t defined in <ucontext.h> and the four func‐
       tions getcontext(), setcontext(), makecontext(3), and swapcontext(3) that allow user-level context switching between  multi‐
       ple 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

           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 to by ucp to the currently active context.

       The  function  setcontext()  restores  the  user context pointed to by ucp.  A successful call does not return.  The context
       should have been obtained by a call of getcontext(), or makecontext(3), or received as the third argument to a  signal  han‐
       dler (see the discussion of the SA_SIGINFO flag in sigaction(2)).

       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 speci‐
       fied 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".

       When successful, getcontext() returns 0 and setcontext() does not return.  On error, both return -1 and set errno  to  indi‐
       cate the error.

       None defined.

       For an explanation of the terms used in this section, see attributes(7).

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

       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.

       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 con‐
       trol.  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 setcon‐
       text() instead.

       sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3), makecontext(3), sigsetjmp(3), signal(7)

Linux man-pages 6.03                                         2022-12-15                                               getcontext(3)