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 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.
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).
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)