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   rt_sigreturn    ( 2 )

возврат из обработчика сигнала и фрейм стека очистки (return from signal handler and cleanup stack frame)

Имя (Name)

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

Синопсис (Synopsis)

int sigreturn(...);

Описание (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.


Возвращаемое значение (Return value)

sigreturn() never returns.

Стандарты (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.

Примечание (Note)

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.


Смотри также (See also)

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