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man: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  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.

RETURN VALUE

     sigreturn() never returns.

CONFORMING TO

     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.

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 loca-
     tion  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 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_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 4.16 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)

/data/webs/external/dokuwiki/data/pages/man/sigreturn.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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