1 GETCONTEXT(2)                    System Calls                    GETCONTEXT(2)
   2 
   3 NAME
   4      getcontext, getcontext_extd, setcontext - get and set current user
   5      context
   6 
   7 SYNOPSIS
   8      #include <ucontext.h>
   9 
  10      int
  11      getcontext(ucontext_t *ucp);
  12 
  13      int
  14      getcontext_extd(ucontext_t *ucp, uint32_t flags);
  15 
  16      int
  17      setcontext(const ucontext_t *ucp);
  18 
  19 DESCRIPTION
  20      The getcontext() function initializes the structure pointed to by ucp to
  21      the current user context of the calling process.  The ucontext_t type
  22      that ucp points to defines the user context and includes the contents of
  23      the calling process' machine registers, the signal mask, and the current
  24      execution stack.
  25 
  26      The ucontext_t structure is a part of the system ABI.  However, most
  27      architectures have added additional register states such as the extended
  28      vector and floating point registers that are not part of that.  To
  29      facilitate getting that state (such as the x86 xsave area) the
  30      getcontext_extd() function exists.  Once called, the context will be
  31      initialized and is suitable for use in other context operations just as
  32      though one had called getcontext().
  33 
  34      Unlike the getcontext() function, getcontext_extd() assumes that callers
  35      have previously initialized ucp and thus it treats additional members
  36      (such as the uc_xsave member on x86) as potentially valid.  To allow for
  37      all extended states to be copied out, ucp must be allocated with
  38      ucontext_alloc(3C).  Otherwise whether it is declared on the stack, as
  39      global data, allocated dynamically, or part of a structure, ucp must be
  40      zeroed through a call to bzero(3C) or memset(3C) prior to calling
  41      getcontext_extd().  Improper initialization can lead to memory safety
  42      bugs, making it critical that this is done.
  43 
  44      The flags member must be zero and is present to allow for what is copied
  45      out to change in the future.  This indicates that the system should
  46      attempt to copy out all extended states, though if the ucontext_t was not
  47      allocated with ucontext_alloc(3C), some extended states may not be.
  48 
  49      The setcontext() function restores the user context pointed to by ucp.  A
  50      successful call to setcontext() does not return; program execution
  51      resumes at the point specified by the ucp argument passed to
  52      setcontext().  The ucp argument should be created either by a prior call
  53      to getcontext(), or by being passed as an argument to a signal handler.
  54      If the ucp argument was created with getcontext(), program execution
  55      continues as if the corresponding call of getcontext() had just returned.
  56      If the ucp argument was created with makecontext(3C), program execution
  57      continues with the function passed to makecontext(3C).  When that
  58      function returns, the process continues as if after a call to
  59      setcontext() with the ucp argument that was input to makecontext(3C).  If
  60      the ucp argument was passed to a signal handler, program execution
  61      continues with the program instruction following the instruction
  62      interrupted by the signal.  If the uc_link member of the ucontext_t
  63      structure pointed to by the ucp argument is NULL, then this context is
  64      the main context, and the process will exit when this context returns.
  65      The effects of passing a ucp argument obtained from any other source are
  66      unspecified.
  67 
  68 RETURN VALUES
  69      On successful completion, setcontext() does not return and getcontext()
  70      and getcontext_extd() returns 0.  Otherwise, -1 is returned.
  71 
  72 ERRORS
  73      No errors are defined for getcontext() or setcontext().
  74 
  75      The getcontext_extd() function only sets errno in some circumstances when
  76      it fails.  The function may fail if:
  77 
  78      EINVAL             flags had invalid values.
  79 
  80 USAGE
  81      When a signal handler is executed, the current user context is saved and
  82      a new context is created.  If the thread leaves the signal handler via
  83      longjmp(3C), then it is unspecified whether the context at the time of
  84      the corresponding setjmp(3C) call is restored and thus whether future
  85      calls to getcontext() will provide an accurate representation of the
  86      current context, since the context restored by longjmp(3C) may not
  87      contain all the information that setcontext() requires.  Signal handlers
  88      should use siglongjmp(3C) instead.
  89 
  90      Portable applications should not modify or access the uc_mcontext member
  91      of ucontext_t.  A portable application cannot assume that context
  92      includes any process-wide static data, possibly including errno.  Users
  93      manipulating contexts should take care to handle these explicitly when
  94      required.
  95 
  96 INTERFACE STABILITY
  97      Committed
  98 
  99 SEE ALSO
 100      sigaction(2), sigaltstack(2), sigprocmask(2), bsd_signal(3C),
 101      makecontext(3C), setjmp(3C), sigsetjmp(3C), ucontext_alloc(3C),
 102      ucontext.h(3HEAD), attributes(7), standards(7)
 103 
 104 illumos                        January 24, 2022                        illumos