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 set-
tings) 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 "sig-
nal 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 equiva-
lent. 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 struc-
ture (see <sys/ucontext.h>). That structure is visible within the sig-
nal handler as the third argument of a handler established with the
On some other UNIX systems, the operation of the signal trampoline dif-
fers a little. In particular, on some systems, upon transitioning back
to user mode, the kernel passes control to the trampoline (rather than
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latest version of this page, can be found at
Linux 2015-12-28 SIGRETURN(2)
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