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 han-
       dler, 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 4.15 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)