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+.\"	$NetBSD: ucontext.2,v 1.10 2018/02/27 10:09:42 uwe Exp $
+.\"
+.\" Copyright (c) 2001 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This code is derived from software contributed to The NetBSD Foundation
+.\" by Klaus Klein.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd February 25, 2018
+.Dt UCONTEXT 2
+.Os
+.Sh NAME
+.Nm ucontext
+.Nd user context
+.Sh SYNOPSIS
+.In ucontext.h
+.Fn _UC_MACHINE_SP "&uc"
+.Fn _UC_MACHINE_FP "&uc"
+.Fn _UC_MACHINE_PC "&uc"
+.Fn _UC_MACHINE_INTRV "&uc"
+.Fn _UC_MACHINE_SET_PC "&uc"
+.Sh DESCRIPTION
+The
+.Vt ucontext_t
+is a structure type which is used to describe the context of a
+thread of control within the execution of a process.
+A thread's context includes its stack, saved registers, and list of
+blocked signals.
+.Pp
+The
+.Vt ucontext_t
+structure includes the following members:
+.Bd -literal -offset indent
+ucontext_t *uc_link
+sigset_t    uc_sigmask
+stack_t     uc_stack
+mcontext_t  uc_mcontext
+.Ed
+.Pp
+The
+.Fa uc_link
+member points to the context that will be resumed after the function
+specified when modifying a context using
+.Xr makecontext 3
+has returned.
+If
+.Fa uc_link
+is a
+.Dv NULL
+pointer, then the context is the main context,
+and the process will exit with an exit status of 0 upon return.
+.Pp
+The
+.Fa uc_sigmask
+member is the set of signals that are blocked when the context is
+activated.
+Further information can be found in
+.Xr sigprocmask 2 .
+.Pp
+The
+.Fa uc_stack
+member defines the stack used by the context.
+Further information can be found in
+.Xr sigaltstack 2 .
+.Pp
+The
+.Fa uc_mcontext
+member defines the machine state associated with the context;
+it may consist of general registers, floating point registers
+and other machine-specific information.
+Its description is beyond the scope of this manual page;
+portable applications should not access this structure member.
+.Pp
+The following auxiliary macros are available to access some of that
+machine-specific information:
+.Bl -tag -width ".Fn _UC_MACHINE_SET_PC" -offset indent
+.It Fn _UC_MACHINE_SP
+\(em read stack pointer,
+.It Fn _UC_MACHINE_FP
+\(em read frame pointer (base pointer),
+.It Fn _UC_MACHINE_PC
+\(em read program counter,
+.It Fn _UC_MACHINE_INTRV
+\(em read integer return value,
+.It Fn _UC_MACHINE_SET_PC
+\(em write program counter.
+.El
+.Pp
+The frame pointer macro does not guarantee to retrieve a reliable value
+and should not be used in a code unless no other debugging format is
+easily accessible.
+A compiler might optimize the frame pointer register in a function,
+reusing it as a general purpose register storage
+.Fl ( fomit-frame-pointer )
+or emit function prologues only before parts that need them
+.Fl ( fshrink-wrap ) .
+.Sh SEE ALSO
+.Xr _exit 2 ,
+.Xr getcontext 2 ,
+.Xr setcontext 2 ,
+.Xr sigaltstack 2 ,
+.Xr sigprocmask 2 ,
+.Xr makecontext 3 ,
+.Xr swapcontext 3
+.Sh STANDARDS
+The
+.Vt ucontext_t
+type conforms to
+.St -xsh5
+and
+.St -p1003.1-2001 .
+The
+.St -p1003.1-2008
+revision removed the
+.Vt ucontext_t
+from the specification.