-.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
+.\" Copyright (C) 2008, 2014, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" Created Sat Aug 21 1995 Thomas K. Dyas <tdyas@eden.rutgers.edu>
.\" Modified Tue Oct 22 22:09:03 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" 2008-06-26, mtk, added some more detail on the work done by sigreturn()
+.\" 2014-12-05, mtk, rewrote all of the rest of the original page
.\"
-.TH SIGRETURN 2 2013-07-30 "Linux" "Linux Programmer's Manual"
+.TH SIGRETURN 2 2014-12-31 "Linux" "Linux Programmer's Manual"
.SH NAME
sigreturn \- return from signal handler and cleanup stack frame
.SH SYNOPSIS
-.BI "int sigreturn(unsigned long " __unused );
+.BI "int sigreturn(...);"
.SH DESCRIPTION
-When the Linux kernel creates the stack frame for a signal handler, a
-call to
-.BR sigreturn ()
-is inserted into the stack frame so that upon
-return from the signal handler,
-.BR sigreturn ()
-will be called.
+If the Linux kernel determines that an unblocked
+signal is pending for a process, then,
+at the next transition back to user mode in that process
+(e.g., upon return from a system call or
+when the process is rescheduled onto the CPU),
+it saves various pieces of process context
+(processor status word, registers, signal mask, and signal stack settings)
+into the user-space stack.
+.\" See arch/x86/kernel/signal.c::__setup_frame() [in 3.17 source code]
+
+The kernel also arranges that, during the transition back to user mode,
+the signal handler is called, and that, upon return from the handler,
+control passes to a piece of user-space code commonly called
+the "signal trampoline".
+The signal trampoline code in turn calls
+.BR sigreturn ().
This
.BR sigreturn ()
call undoes everything that was
-done\(emchanging the process's signal mask, switching stacks (see
+done\(emchanging the process's signal mask, switching signal stacks (see
.BR sigaltstack "(2))\(emin "
-order to invoke the signal handler:
-it restores the process's signal mask, switches stacks,
-and restores the process's context (registers, processor flags),
-so that the process directly resumes execution
+order to invoke the signal handler.
+It restores the process's signal mask, switches stacks,
+and restores the process's context
+(processor flags and registers,
+including the stack pointer and instruction pointer),
+so that the process resumes execution
at the point where it was interrupted by the signal.
.SH RETURN VALUE
.BR sigreturn ()
never returns.
-.SH FILES
-/usr/src/linux/arch/i386/kernel/signal.c
-.br
-/usr/src/linux/arch/alpha/kernel/entry.S
.SH CONFORMING TO
+Many UNIX-type systems have a
.BR sigreturn ()
-is specific to Linux and should not be used in programs intended to be
-portable.
+system call or near equivalent.
+However, this call is not specified in POSIX,
+and details of its behavior vary across systems.
.SH NOTES
-The
.BR sigreturn ()
-call is used by the kernel to implement signal handlers.
+exists only to allow the implementation of signal handlers.
It should
.B never
be called directly.
-Better yet, the specific use of the
-.I __unused
-argument varies depending on the architecture.
+Details of the arguments (if any) passed to
+.BR sigreturn ()
+vary depending on the architecture.
+
+Once upon a time, UNIX systems placed the signal trampoline code
+onto the user stack.
+Nowadays, pages of the user stack are protected so as to
+disallow code execution.
+Thus, on contemporary Linux systems, depending on the architecture,
+the signal trampoline code lives either in the
+.BR vdso (7)
+or in the C library.
+In the latter case,
+.\" See, for example, sysdeps/unix/sysv/linux/i386/sigaction.c and
+.\" sysdeps/unix/sysv/linux/x86_64/sigaction.c in the glibc (2.20) source.
+the C library supplies the location of the trampoline code using the
+.I sa_restorer
+field of the
+.I sigaction
+structure that is passed to
+.BR sigaction (2),
+and sets the
+.BR SA_RESTORER
+flag in the
+.IR sa_flags
+field.
+
+The saved process context information is placed in a
+.I ucontext_t
+structure (see
+.IR <sys/ucontext.h> ).
+That structure is visible within the signal handler
+as the third argument of a handler established with the
+.BR SA_SIGINFO
+flag.
+
+On some other UNIX systems,
+the operation of the signal trampoline differs a little.
+In particular, on some systems, upon transitioning back to user mode,
+the kernel passes control to the trampoline (rather than the signal handler),
+and the trampoline code calls the signal handler (and then calls
+.BR sigreturn ()
+once the handler returns).
.SH SEE ALSO
.BR kill (2),
.BR restart_syscall (2),
.BR sigaltstack (2),
.BR signal (2),
+.BR getcontext (3),
.BR signal (7)
.SH COLOPHON
-This page is part of release 3.65 of the Linux
+This page is part of release 3.79 of the Linux
.I man-pages
project.
A description of the project,
-and information about reporting bugs,
+information about reporting bugs,
+and the latest version of this page,
can be found at
\%http://www.kernel.org/doc/man\-pages/.
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