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SIGRETURN(2) Linux Programmer's Manual SIGRETURN(2)
NAME
sigreturn, rt_sigreturn - return from signal handler and cleanup stack frame
SYNOPSIS
int sigreturn(...);
DESCRIPTION
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 re‐
turn 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 re‐
stores 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.
RETURN VALUE
sigreturn() never returns.
CONFORMING TO
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.
NOTES
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 ar‐
chitectures, 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 ex‐
ecution. 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.
SEE ALSO
kill(2), restart_syscall(2), sigaltstack(2), signal(2), getcontext(3), signal(7), vdso(7)
Linux 2021-03-22 SIGRETURN(2)