.\"
.\" See also Section 11.3.3 of Understanding the Linux Kernel, 3rd edition
.\"
-.TH RESTART_SYSCALL 2 2014-02-11 "Linux" "Linux Programmer's Manual"
+.TH RESTART_SYSCALL 2 2014-12-31 "Linux" "Linux Programmer's Manual"
.SH NAME
restart_syscall \- restart a system call after interruption by a stop signal
.SH SYNOPSIS
.\" These system calls correspond to the special internal errno value
.\" ERESTART_RESTARTBLOCK. Each of the system calls has a "restart"
.\" helper function that is invoked by restart_syscall().
+.\" Notable (as at Linux 3.17) is that poll() has such a "restart"
+.\" function, but ppoll(), select(), and pselect() do not.
+.\" This means that the latter system calls do not take account of the
+.\" time spent in the stopped state when restarting.
.BR restart_syscall ()
restarts the interrupted system call with a
time argument that is suitably adjusted to account for the
was stopped by a signal).
Without the
.BR restart_syscall ()
-mechanism, restarting these system calls would not correctly deduce the
+mechanism, restarting these system calls would not correctly deduct the
already elapsed time when the process continued execution.
.SH RETURN VALUE
The return value of
because it is intended for use only by the kernel and
should never be called by applications.
+The kernel uses
+.BR restart_syscall ()
+to ensure that when a system call is restarted
+after a process has been stopped by a signal and then resumed by
+.BR SIGCONT ,
+then the time that the process spent in the stopped state is counted
+against the timeout interval specified in the original system call.
+In the case of system calls that take a timeout argument and
+automatically restart after a stop signal plus
+.BR SIGCONT ,
+but which do not have the
+.BR restart_syscall (2)
+mechanism built in, then, after the process resumes execution,
+the time that the process spent in the stop state is
+.I not
+counted against the timeout value.
+Notable examples of system calls that suffer this problem are
+.BR ppoll (2),
+.BR select (2),
+and
+.BR pselect (2).
+
From user space, the operation of
.BR restart_syscall (2)
is largely invisible:
to the process that made the system call that is restarted,
it appears as though that system call executed and
returned in the usual fashion.
-.\"
-.\" FIXME
-.\" There is one oddness in the implementation though, with respect to
-.\" nanosleep() (and probably also clock_nanosleep()). The scenario
-.\" is as follows:
-.\" 1. Start a nanosleep() for (say) 30 seconds,
-.\" 2. Stop the process with (say) SIGTSTP (^Z).
-.\" 3. Resume the process with SIGCONT,
-.\" 4. Upon return, the 'rem' argument of nanosleep() will contain the
-.\" remaining unslept time **at the time when SIGTSTP was delivered**.
-.\" The behavior at point 4 is odd, but doesn't violate the standards, which
-.\" specify the treatment of 'rem' only when the system call returns with
-.\" the error EINTR (i.e., the call was interrupted by a signal handler).
-.\"
.SH SEE ALSO
.BR sigaction (2),
.BR sigreturn (2),
.BR signal (7)
-.\" FIXME select(2) should probably get the restart_syscall() treatment:
+.\" FIXME . ppoll(2), select(2), and pselect(2)
+.\" should probably get the restart_syscall() treatment:
.\" If a select() call is suspended by stop-sig+SIGCONT, the time
.\" spent suspended is *not* deducted when the select() is restarted.
-.\" FIXME: check whether recvmmsg() handles stop-sig+SIGCONT properly.
+.\" FIXME . check whether recvmmsg() handles stop-sig+SIGCONT properly.
.SH COLOPHON
-This page is part of release 3.64 of the Linux
+This page is part of release 3.79 of the Linux
.I man-pages
project.
A description of the project,
-and information about reporting bugs,
+information about reporting bugs,
+and the latest version of this page,
can be found at
\%http://www.kernel.org/doc/man\-pages/.
OSDN Git Service
