SIGRETURN(2) Linux Programmer's Manual SIGRETURN(2)
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 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 han-
dler, control passes to a piece of user-space code commonly called the "signal trampoline". The signal trampoline code in turn calls
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
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)
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Linux 2017-09-15 SIGRETURN(2)