SIGRETURN(2)               Linux Programmer's Manual              SIGRETURN(2)

       sigreturn,  rt_sigreturn - return from signal handler and cleanup stack

       int sigreturn(...);

       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  sta-
       tus 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 "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.  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 con-
       text  (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.   (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,  de-
       pending 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

       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  via  sigac-
       tion(2) with the SA_SIGINFO flag.

       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
       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_si-
       greturn() when the kernel provides it.

       kill(2),  restart_syscall(2), sigaltstack(2), signal(2), getcontext(3),
       signal(7), vdso(7)

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       description  of  the project, information about reporting bugs, and the
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Linux                             2017-09-15                      SIGRETURN(2)
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