sigreturn, rt_sigreturn - return from signal handler and cleanup stack frame
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
() call undoes everything that was done—changing the
process's signal mask, switching signal stacks (see
)—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.
() 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
() 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
flag in the sa_flags
The saved process context information is placed in a ucontext_t
). That structure is visible within the
signal handler as the third argument of a handler established with the
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
() 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,
() was added to support an enlarged sigset_t
The GNU C library hides these details from us, transparently employing
() when the kernel provides it.
This page is part of release 4.10 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at