rt_sigreturn − return from signal handler and cleanup
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
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().
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.
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.
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
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
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.
signal(2), getcontext(3), signal(7),
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