NAME

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

LIBRARY

       Standard C library (libc, -lc)

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.

VERSIONS

       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.

STANDARDS

       None.

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)

Linux man-pages 6.15                               2025-05-17                                       sigreturn(2)