236 lines
6.0 KiB
HTML
236 lines
6.0 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 SIGRETURN</TITLE>
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</HEAD><BODY>
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<H1>SIGRETURN</H1>
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Section: Linux Programmer's Manual (2)<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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sigreturn, rt_sigreturn - return from signal handler and cleanup stack frame
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>int sigreturn(...);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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If the Linux kernel determines that an unblocked
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signal is pending for a process, then,
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at the next transition back to user mode in that process
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(e.g., upon return from a system call or
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when the process is rescheduled onto the CPU),
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it creates a new frame on the user-space stack where it
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saves various pieces of process context
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(processor status word, registers, signal mask, and signal stack settings).
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<P>
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The kernel also arranges that, during the transition back to user mode,
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the signal handler is called, and that, upon return from the handler,
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control passes to a piece of user-space code commonly called
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the "signal trampoline".
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The signal trampoline code in turn calls
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<B>sigreturn</B>().
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<P>
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This
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<B>sigreturn</B>()
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call undoes everything that was
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done---changing the process's signal mask, switching signal stacks (see
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<B><A HREF="/cgi-bin/man/man2html?2+sigaltstack">sigaltstack</A></B>(2))---in
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order to invoke the signal handler.
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Using the information that was earlier saved on the user-space stack
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<B>sigreturn</B>()
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restores the process's signal mask, switches stacks,
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and restores the process's context
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(processor flags and registers,
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including the stack pointer and instruction pointer),
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so that the process resumes execution
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at the point where it was interrupted by the signal.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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<B>sigreturn</B>()
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never returns.
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<A NAME="lbAF"> </A>
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<H2>CONFORMING TO</H2>
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Many UNIX-type systems have a
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<B>sigreturn</B>()
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system call or near equivalent.
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However, this call is not specified in POSIX,
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and details of its behavior vary across systems.
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<A NAME="lbAG"> </A>
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<H2>NOTES</H2>
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<B>sigreturn</B>()
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exists only to allow the implementation of signal handlers.
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It should
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<B>never</B>
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be called directly.
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(Indeed, a simple
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<B>sigreturn</B>()
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wrapper in the GNU C library simply returns -1, with
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<I>errno</I>
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set to
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<B>ENOSYS</B>.)
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Details of the arguments (if any) passed to
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<B>sigreturn</B>()
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vary depending on the architecture.
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(On some architectures, such as x86-64,
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<B>sigreturn</B>()
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takes no arguments, since all of the information that it requires
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is available in the stack frame that was previously created by the
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kernel on the user-space stack.)
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<P>
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Once upon a time, UNIX systems placed the signal trampoline code
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onto the user stack.
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Nowadays, pages of the user stack are protected so as to
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disallow code execution.
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Thus, on contemporary Linux systems, depending on the architecture,
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the signal trampoline code lives either in the
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<B><A HREF="/cgi-bin/man/man2html?7+vdso">vdso</A></B>(7)
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or in the C library.
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In the latter case,
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the C library's
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2)
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wrapper function informs the kernel of the location of the trampoline code
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by placing its address in the
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<I>sa_restorer</I>
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field of the
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<I>sigaction</I>
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structure,
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and sets the
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<B>SA_RESTORER</B>
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flag in the
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<I>sa_flags</I>
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field.
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<P>
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The saved process context information is placed in a
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<I>ucontext_t</I>
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structure (see
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<I><<A HREF="file:///usr/include/sys/ucontext.h">sys/ucontext.h</A>></I>).
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That structure is visible within the signal handler
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as the third argument of a handler established via
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2)
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with the
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<B>SA_SIGINFO</B>
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flag.
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<P>
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On some other UNIX systems,
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the operation of the signal trampoline differs a little.
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In particular, on some systems, upon transitioning back to user mode,
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the kernel passes control to the trampoline (rather than the signal handler),
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and the trampoline code calls the signal handler (and then calls
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<B>sigreturn</B>()
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once the handler returns).
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<A NAME="lbAH"> </A>
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<H3>C library/kernel differences</H3>
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The original Linux system call was named
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<B>sigreturn</B>().
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However, with the addition of real-time signals in Linux 2.2,
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a new system call,
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<B>rt_sigreturn</B>()
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was added to support an enlarged
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<I>sigset_t</I>
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type.
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The GNU C library
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hides these details from us, transparently employing
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<B>rt_sigreturn</B>()
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when the kernel provides it.
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<A NAME="lbAI"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+kill">kill</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+restart_syscall">restart_syscall</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+signal">signal</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+getcontext">getcontext</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?7+signal">signal</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+vdso">vdso</A></B>(7)
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<A NAME="lbAJ"> </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">CONFORMING TO</A><DD>
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<DT id="6"><A HREF="#lbAG">NOTES</A><DD>
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<DL>
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<DT id="7"><A HREF="#lbAH">C library/kernel differences</A><DD>
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</DL>
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<DT id="8"><A HREF="#lbAI">SEE ALSO</A><DD>
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<DT id="9"><A HREF="#lbAJ">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: 00:05:34 GMT, March 31, 2021
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</BODY>
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</HTML>
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