.\" Copyright (c) 2013 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" http://thread.gmane.org/gmane.linux.kernel/76552/focus=76803
.\" From: Linus Torvalds <torvalds <at> transmeta.com>
.\" Subject: Re: [PATCH] compatibility syscall layer (lets try again)
.\" Newsgroups: gmane.linux.kernel
.\" Date: 2002-12-05 02:51:12 GMT
.\"
.\" See also Section 11.3.3 of Understanding the Linux Kernel, 3rd edition
.\"
.TH restart_syscall 2 2023-03-30 "Linux man-pages 6.04"
.SH NAME
restart_syscall \- restart a system call after interruption by a stop signal
.SH SYNOPSIS
.nf
.B long restart_syscall(void);
.fi
.PP
.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
The
.BR restart_syscall ()
system call is used to restart certain system calls
after a process that was stopped by a signal (e.g.,
.B SIGSTOP
or
.BR SIGTSTP )
is later resumed after receiving a
.B SIGCONT
signal.
This system call is designed only for internal use by the kernel.
.PP
.BR restart_syscall ()
is used for restarting only those system calls that,
when restarted, should adjust their time-related parameters\[em]namely
.BR poll (2)
(since Linux 2.6.24),
.BR nanosleep (2)
(since Linux 2.6),
.BR clock_nanosleep (2)
(since Linux 2.6),
and
.BR futex (2),
when employed with the
.B FUTEX_WAIT
(since Linux 2.6.22)
and
.B FUTEX_WAIT_BITSET
(since Linux 2.6.31)
operations.
.\" 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
time that has already elapsed (including the time where the process
was stopped by a signal).
Without the
.BR restart_syscall ()
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
.BR restart_syscall ()
is the return value of whatever system call is being restarted.
.SH ERRORS
.I errno
is set as per the errors for whatever system call is being restarted by
.BR restart_syscall ().
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.6.
.SH NOTES
There is no glibc wrapper for this system call,
because it is intended for use only by the kernel and
should never be called by applications.
.PP
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 ()
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).
.PP
From user space, the operation of
.BR restart_syscall ()
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.
.SH SEE ALSO
.BR sigaction (2),
.BR sigreturn (2),
.BR signal (7)
.\" 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.