sigreturn

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 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.

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 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).

   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)

COLOPHON
       This page is part of release 5.03 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2017-09-15                      SIGRETURN(2)