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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
SIGRETURN(2) Linux Programmer's Manual SIGRETURN(2)
sigreturn, rt_sigreturn - return from signal handler and cleanup
stack frame
int sigreturn(...);
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.
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
sigreturn().
This sigreturn() call undoes everything that was done—changing the
process's signal mask, switching signal stacks (see
sigaltstack(2))—in 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.
sigreturn() never returns.
Many UNIX-type systems have a sigreturn() system call or near
equivalent. However, this call is not specified in POSIX, and
details of its behavior vary across systems.
sigreturn() exists only to allow the implementation of signal
handlers. It should never be called directly. Details of the
arguments (if any) passed to 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 vdso(7) or in the C library. In the latter case, the C
library supplies the location of the trampoline code using the
sa_restorer field of the sigaction structure that is passed to
sigaction(2), and sets the SA_RESTORER flag in the sa_flags field.
The saved process context information is placed in a ucontext_t
structure (see <sys/ucontext.h>). That structure is visible within
the signal handler as the third argument of a handler established
with the 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 sigreturn() once the handler returns).
C library/kernel differences
The original Linux system call was named sigreturn(). However, with
the addition of real-time signals in Linux 2.2, a new system call,
rt_sigreturn() was added to support an enlarged sigset_t type. The
GNU C library hides these details from us, transparently employing
rt_sigreturn() when the kernel provides it.
kill(2), restart_syscall(2), sigaltstack(2), signal(2),
getcontext(3), signal(7), vdso(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2015-12-28 SIGRETURN(2)
Pages that refer to this page: prctl(2), restart_syscall(2), seccomp(2), sigaction(2), syscalls(2), signal(7)