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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | STANDARDS | NOTES | SEE ALSO | COLOPHON |
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sigreturn(2) System Calls Manual sigreturn(2)
sigreturn, rt_sigreturn - return from signal handler and cleanup
stack frame
Standard C library (libc, -lc)
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 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).
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. Using the
information that was earlier saved on the user-space stack
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.
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.
None.
sigreturn() exists only to allow the implementation of signal
handlers. It should never be called directly. (Indeed, a simple
sigreturn() wrapper in the GNU C library simply returns -1, with
errno set to ENOSYS.) Details of the arguments (if any) passed to
sigreturn() vary depending on the architecture. (On some
architectures, such as x86-64, 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.)
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's sigaction(2) wrapper function
informs the kernel of the location of the trampoline code by
placing its address in the sa_restorer field of the sigaction
structure, 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 via sigaction(2) 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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 sigreturn(2)
Pages that refer to this page: PR_SET_SYSCALL_USER_DISPATCH(2const), restart_syscall(2), seccomp(2), sigaction(2), syscalls(2), signal(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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