mirror of https://github.com/mkerrisk/man-pages
241 lines
6.6 KiB
Groff
241 lines
6.6 KiB
Groff
.\" Copyright (C) 2001 Andries Brouwer (aeb@cwi.nl)
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.\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" 2006-08-02, mtk, Added example program
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.\"
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.TH MAKECONTEXT 3 2020-12-21 "GNU" "Linux Programmer's Manual"
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.SH NAME
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makecontext, swapcontext \- manipulate user context
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.SH SYNOPSIS
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.B #include <ucontext.h>
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.PP
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.BI "void makecontext(ucontext_t *" ucp ", void (*" func )(),
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.BI "int " argc ", ...);"
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.PP
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.BI "int swapcontext(ucontext_t *" oucp ", const ucontext_t *" ucp );
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.SH DESCRIPTION
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In a System V-like environment, one has the type
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.I ucontext_t
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(defined in
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.I <ucontext.h>
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and described in
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.BR getcontext (3))
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and the four functions
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.BR getcontext (3),
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.BR setcontext (3),
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.BR makecontext (),
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and
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.BR swapcontext ()
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that allow user-level context switching
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between multiple threads of control within a process.
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.PP
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The
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.BR makecontext ()
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function modifies the context pointed to
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by \fIucp\fP (which was obtained from a call to
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.BR getcontext (3)).
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Before invoking
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.BR makecontext (),
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the caller must allocate a new stack
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for this context and assign its address to \fIucp\->uc_stack\fP,
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and define a successor context and
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assign its address to \fIucp\->uc_link\fP.
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.PP
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When this context is later activated (using
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.BR setcontext (3)
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or
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.BR swapcontext ())
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the function \fIfunc\fP is called,
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and passed the series of integer
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.RI ( int )
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arguments that follow
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.IR argc ;
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the caller must specify the number of these arguments in
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.IR argc .
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When this function returns, the successor context is activated.
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If the successor context pointer is NULL, the thread exits.
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.PP
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The
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.BR swapcontext ()
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function saves the current context in
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the structure pointed to by \fIoucp\fP, and then activates the
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context pointed to by \fIucp\fP.
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.SH RETURN VALUE
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When successful,
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.BR swapcontext ()
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does not return.
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(But we may return later, in case \fIoucp\fP is
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activated, in which case it looks like
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.BR swapcontext ()
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returns 0.)
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On error,
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.BR swapcontext ()
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returns \-1 and
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sets \fIerrno\fP appropriately.
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.SH ERRORS
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.TP
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.B ENOMEM
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Insufficient stack space left.
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.SH VERSIONS
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.BR makecontext ()
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and
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.BR swapcontext ()
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are provided in glibc since version 2.1.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lb lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR makecontext ()
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T} Thread safety MT-Safe race:ucp
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T{
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.BR swapcontext ()
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T} Thread safety MT-Safe race:oucp race:ucp
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.TE
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.SH CONFORMING TO
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SUSv2, POSIX.1-2001.
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POSIX.1-2008 removes the specifications of
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.BR makecontext ()
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and
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.BR swapcontext (),
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citing portability issues, and
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recommending that applications be rewritten to use POSIX threads instead.
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.SH NOTES
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The interpretation of \fIucp\->uc_stack\fP is just as in
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.BR sigaltstack (2),
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namely, this struct contains the start and length of a memory area
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to be used as the stack, regardless of the direction of growth of
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the stack.
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Thus, it is not necessary for the user program to
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worry about this direction.
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.PP
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On architectures where
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.I int
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and pointer types are the same size
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(e.g., x86-32, where both types are 32 bits),
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you may be able to get away with passing pointers as arguments to
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.BR makecontext ()
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following
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.IR argc .
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However, doing this is not guaranteed to be portable,
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is undefined according to the standards,
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and won't work on architectures where pointers are larger than
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.IR int s.
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Nevertheless, starting with version 2.8, glibc makes some changes to
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.BR makecontext (),
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to permit this on some 64-bit architectures (e.g., x86-64).
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.SH EXAMPLES
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The example program below demonstrates the use of
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.BR getcontext (3),
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.BR makecontext (),
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and
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.BR swapcontext ().
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Running the program produces the following output:
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.PP
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.in +4n
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.EX
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.RB "$" " ./a.out"
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main: swapcontext(&uctx_main, &uctx_func2)
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func2: started
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func2: swapcontext(&uctx_func2, &uctx_func1)
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func1: started
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func1: swapcontext(&uctx_func1, &uctx_func2)
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func2: returning
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func1: returning
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main: exiting
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.EE
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.in
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.SS Program source
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\&
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.EX
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#include <ucontext.h>
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#include <stdio.h>
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#include <stdlib.h>
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static ucontext_t uctx_main, uctx_func1, uctx_func2;
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#define handle_error(msg) \e
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do { perror(msg); exit(EXIT_FAILURE); } while (0)
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static void
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func1(void)
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{
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printf("func1: started\en");
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printf("func1: swapcontext(&uctx_func1, &uctx_func2)\en");
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if (swapcontext(&uctx_func1, &uctx_func2) == \-1)
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handle_error("swapcontext");
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printf("func1: returning\en");
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}
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static void
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func2(void)
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{
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printf("func2: started\en");
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printf("func2: swapcontext(&uctx_func2, &uctx_func1)\en");
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if (swapcontext(&uctx_func2, &uctx_func1) == \-1)
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handle_error("swapcontext");
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printf("func2: returning\en");
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}
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int
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main(int argc, char *argv[])
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{
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char func1_stack[16384];
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char func2_stack[16384];
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if (getcontext(&uctx_func1) == \-1)
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handle_error("getcontext");
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uctx_func1.uc_stack.ss_sp = func1_stack;
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uctx_func1.uc_stack.ss_size = sizeof(func1_stack);
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uctx_func1.uc_link = &uctx_main;
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makecontext(&uctx_func1, func1, 0);
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if (getcontext(&uctx_func2) == \-1)
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handle_error("getcontext");
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uctx_func2.uc_stack.ss_sp = func2_stack;
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uctx_func2.uc_stack.ss_size = sizeof(func2_stack);
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/* Successor context is f1(), unless argc > 1 */
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uctx_func2.uc_link = (argc > 1) ? NULL : &uctx_func1;
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makecontext(&uctx_func2, func2, 0);
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printf("main: swapcontext(&uctx_main, &uctx_func2)\en");
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if (swapcontext(&uctx_main, &uctx_func2) == \-1)
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handle_error("swapcontext");
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printf("main: exiting\en");
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exit(EXIT_SUCCESS);
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}
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.EE
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.SH SEE ALSO
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.BR sigaction (2),
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.BR sigaltstack (2),
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.BR sigprocmask (2),
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.BR getcontext (3),
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.BR sigsetjmp (3)
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