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)
