308 lines
7.4 KiB
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308 lines
7.4 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of GETCONTEXT</TITLE>
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</HEAD><BODY>
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<H1>GETCONTEXT</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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getcontext, setcontext - get or set the user context
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:///usr/include/ucontext.h">ucontext.h</A>></B>
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<P>
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<B>int getcontext(ucontext_t *</B><I>ucp</I><B>);</B>
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<BR>
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<B>int setcontext(const ucontext_t *</B><I>ucp</I><B>);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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In a System V-like environment, one has the two types
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<I>mcontext_t</I>
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and
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<I>ucontext_t</I>
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defined in
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<I><<A HREF="file:///usr/include/ucontext.h">ucontext.h</A>></I>
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and the four functions
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<B>getcontext</B>(),
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<B>setcontext</B>(),
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<B><A HREF="/cgi-bin/man/man2html?3+makecontext">makecontext</A></B>(3),
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and
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<B><A HREF="/cgi-bin/man/man2html?3+swapcontext">swapcontext</A></B>(3)
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that allow user-level context switching between multiple
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threads of control within a process.
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<P>
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The
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<I>mcontext_t</I>
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type is machine-dependent and opaque.
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The
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<I>ucontext_t</I>
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type is a structure that has at least
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the following fields:
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<P>
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typedef struct ucontext_t {
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<BR> struct ucontext_t *uc_link;
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<BR> sigset_t uc_sigmask;
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<BR> stack_t uc_stack;
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<BR> mcontext_t uc_mcontext;
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<BR> ...
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} ucontext_t;
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<P>
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with
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<I>sigset_t</I>
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and
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<I>stack_t</I>
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defined in
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<I><<A HREF="file:///usr/include/signal.h">signal.h</A>></I>.
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Here
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<I>uc_link</I>
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points to the context that will be resumed
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when the current context terminates (in case the current context
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was created using
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<B><A HREF="/cgi-bin/man/man2html?3+makecontext">makecontext</A></B>(3)),
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<I>uc_sigmask</I>
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is the
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set of signals blocked in this context (see
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<B><A HREF="/cgi-bin/man/man2html?2+sigprocmask">sigprocmask</A></B>(2)),
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<I>uc_stack</I>
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is the stack used by this context (see
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<B><A HREF="/cgi-bin/man/man2html?2+sigaltstack">sigaltstack</A></B>(2)),
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and
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<I>uc_mcontext</I>
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is the
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machine-specific representation of the saved context,
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that includes the calling thread's machine registers.
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<P>
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The function
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<B>getcontext</B>()
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initializes the structure
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pointed at by
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<I>ucp</I>
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to the currently active context.
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<P>
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The function
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<B>setcontext</B>()
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restores the user context
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pointed at by
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<I>ucp</I>.
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A successful call does not return.
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The context should have been obtained by a call of
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<B>getcontext</B>(),
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or
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<B><A HREF="/cgi-bin/man/man2html?3+makecontext">makecontext</A></B>(3),
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or passed as third argument to a signal
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handler.
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<P>
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If the context was obtained by a call of
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<B>getcontext</B>(),
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program execution continues as if this call just returned.
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<P>
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If the context was obtained by a call of
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<B><A HREF="/cgi-bin/man/man2html?3+makecontext">makecontext</A></B>(3),
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program execution continues by a call to the function
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<I>func</I>
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specified as the second argument of that call to
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<B><A HREF="/cgi-bin/man/man2html?3+makecontext">makecontext</A></B>(3).
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When the function
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<I>func</I>
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returns, we continue with the
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<I>uc_link</I>
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member of the structure
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<I>ucp</I>
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specified as the
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first argument of that call to
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<B><A HREF="/cgi-bin/man/man2html?3+makecontext">makecontext</A></B>(3).
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When this member is NULL, the thread exits.
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<P>
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If the context was obtained by a call to a signal handler,
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then old standard text says that "program execution continues with the
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program instruction following the instruction interrupted
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by the signal".
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However, this sentence was removed in SUSv2,
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and the present verdict is "the result is unspecified".
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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When successful,
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<B>getcontext</B>()
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returns 0 and
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<B>setcontext</B>()
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does not return.
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On error, both return -1 and set
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<I>errno</I>
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appropriately.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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None defined.
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<A NAME="lbAG"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>getcontext</B>(),
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<B>setcontext</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe race:ucp<BR></TD></TR>
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</TABLE>
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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SUSv2, POSIX.1-2001.
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POSIX.1-2008 removes the specification of
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<B>getcontext</B>(),
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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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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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The earliest incarnation of this mechanism was the
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<B><A HREF="/cgi-bin/man/man2html?3+setjmp">setjmp</A></B>(3)/<B><A HREF="/cgi-bin/man/man2html?3+longjmp">longjmp</A></B>(3)
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mechanism.
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Since that does not define
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the handling of the signal context, the next stage was the
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<B><A HREF="/cgi-bin/man/man2html?3+sigsetjmp">sigsetjmp</A></B>(3)/<B><A HREF="/cgi-bin/man/man2html?3+siglongjmp">siglongjmp</A></B>(3)
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pair.
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The present mechanism gives much more control.
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On the other hand,
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there is no easy way to detect whether a return from
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<B>getcontext</B>()
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is from the first call, or via a
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<B>setcontext</B>()
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call.
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The user has to invent their own bookkeeping device, and a register
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variable won't do since registers are restored.
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<P>
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When a signal occurs, the current user context is saved and
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a new context is created by the kernel for the signal handler.
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Do not leave the handler using
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<B><A HREF="/cgi-bin/man/man2html?3+longjmp">longjmp</A></B>(3):
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it is undefined what would happen with contexts.
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Use
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<B><A HREF="/cgi-bin/man/man2html?3+siglongjmp">siglongjmp</A></B>(3)
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or
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<B>setcontext</B>()
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instead.
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<A NAME="lbAJ"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+sigaltstack">sigaltstack</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+sigprocmask">sigprocmask</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+longjmp">longjmp</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+makecontext">makecontext</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sigsetjmp">sigsetjmp</A></B>(3)
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<A NAME="lbAK"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="4"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="5"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="6"><A HREF="#lbAG">ATTRIBUTES</A><DD>
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<DT id="7"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="8"><A HREF="#lbAI">NOTES</A><DD>
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<DT id="9"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="10"><A HREF="#lbAK">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:44 GMT, March 31, 2021
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</BODY>
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</HTML>
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