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-.\" Copyright (C) 2008, 2014, Michael Kerrisk <mtk.manpages@gmail.com>
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.\" 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 (date) "Linux man-pages (unreleased)"
-.SH NAME
-sigreturn, rt_sigreturn \- return from signal handler and cleanup stack frame
-.SH LIBRARY
-Standard C library
-.RI ( libc ", " \-lc )
-.SH SYNOPSIS
-.nf
-.B int sigreturn(...);
-.fi
-.SH DESCRIPTION
-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 creates a new frame on the user-space stack where it
-saves various pieces of process context
-(processor status word, registers, signal mask, and signal stack settings).
-.\" See arch/x86/kernel/signal.c::__setup_frame() [in Linux 3.17 source code]
-.PP
-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 ().
-.PP
-This
-.BR sigreturn ()
-call undoes everything that was
-done\[em]changing the process's signal mask, switching signal stacks (see
-.BR sigaltstack "(2))\[em]in"
-order to invoke the signal handler.
-Using the information that was earlier saved on the user-space stack
-.BR sigreturn ()
-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 VERSIONS
-Many UNIX-type systems have a
-.BR sigreturn ()
-system call or near equivalent.
-However, this call is not specified in POSIX,
-and details of its behavior vary across systems.
-.SH STANDARDS
-None.
-.SH NOTES
-.BR sigreturn ()
-exists only to allow the implementation of signal handlers.
-It should
-.B never
-be called directly.
-(Indeed, a simple
-.BR sigreturn ()
-.\" See sysdeps/unix/sysv/linux/sigreturn.c and
-.\" signal/sigreturn.c in the glibc source
-wrapper in the GNU C library simply returns \-1, with
-.I errno
-set to
-.BR ENOSYS .)
-Details of the arguments (if any) passed to
-.BR sigreturn ()
-vary depending on the architecture.
-(On some architectures, such as x86-64,
-.BR sigreturn ()
-takes no arguments, since all of the information that it requires
-is available in the stack frame that was previously created by the
-kernel on the user-space stack.)
-.PP
-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's
-.BR sigaction (2)
-wrapper function informs the kernel of the location of the trampoline code
-by placing its address in the
-.I sa_restorer
-field of the
-.I sigaction
-structure,
-and sets the
-.B SA_RESTORER
-flag in the
-.I sa_flags
-field.
-.PP
-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 via
-.BR sigaction (2)
-with the
-.B SA_SIGINFO
-flag.
-.PP
-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).
-.\"
-.SS C library/kernel differences
-The original Linux system call was named
-.BR sigreturn ().
-However, with the addition of real-time signals in Linux 2.2,
-a new system call,
-.BR rt_sigreturn ()
-was added to support an enlarged
-.I sigset_t
-type.
-The GNU C library
-hides these details from us, transparently employing
-.BR rt_sigreturn ()
-when the kernel provides it.
-.\"
-.SH SEE ALSO
-.BR kill (2),
-.BR restart_syscall (2),
-.BR sigaltstack (2),
-.BR signal (2),
-.BR getcontext (3),
-.BR signal (7),
-.BR vdso (7)