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       sigreturn, rt_sigreturn - return from signal handler and cleanup stack frame


       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  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

       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 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.


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


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