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restart_syscall.2: New page for restart_syscall(2) system call
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Copyright (c) 2013 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" http://thread.gmane.org/gmane.linux.kernel/76552/focus=76803
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.\" From: Linus Torvalds <torvalds <at> transmeta.com>
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.\" Subject: Re: [PATCH] compatibility syscall layer (lets try again)
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.\" Newsgroups: gmane.linux.kernel
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.\" Date: 2002-12-05 02:51:12 GMT
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.\"
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.\" See also Section 11.3.3 of Understanding the Linux Kernel, 3rd edition
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.\"
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.TH RESTART_SYSCALL 2 2013-07-30 "Linux" "Linux Programmer's Manual"
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.SH NAME
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restart_syscall \- restart a system call after interruption by a stop signal
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.SH SYNOPSIS
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.B int restart_syscall(void);
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.IR Note :
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There is no glibc wrapper for this system call; see NOTES.
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.SH DESCRIPTION
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The
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.BR restart_syscall ()
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system call is used to restart certain system calls
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after a process that was stopped by a signal (e.g.,
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.BR SIGSTOP
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or
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.BR SIGTSTP )
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is later resumed after receiving a
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.BR SIGCONT
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signal.
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This system call is designed only for internal use by the kernel.
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.BR restart_syscall ()
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is used for restarting only those system calls that,
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when restarted, should adjust their time-related parameters\(emnamely
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.BR poll (2)
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(since Linux 2.6.24),
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.BR nanosleep (2)
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(since Linux 2.6),
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.BR clock_nanosleep (2)
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(since Linux 2.6),
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and
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.BR futex (2),
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when employed with the
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.BR FUTEX_WAIT
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(since Linux 2.6.22)
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and
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.BR FUTEX_WAIT_BITSET
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(since Linux 2.6.31)
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operations.
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.\" These system calls correspond to the special internal errno value
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.\" ERESTART_RESTARTBLOCK. Each of the system calls has a "restart"
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.\" helper function that is invoked by restart_syscall().
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.BR restart_syscall ()
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restarts the interrupted system call with a
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time argument that is suitably adjusted to account for the
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time that has already elapsed (including the time where the process
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was stopped by a signal).
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Without the
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.BR restart_syscall ()
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mechanism, restarting these system calls would not correctly deduce the
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already elapsed time when the process continued execution.
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.SH RETURN VALUE
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The return value of
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.BR restart_syscall ()
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is the return value of whatever system call is being restarted.
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.SH ERRORS
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.I errno
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is set as per the errors for whatever system call is being restarted by
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.BR restart_syscall ().
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.SH VERSIONS
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The
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.BR restart_syscall ()
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system call is present since Linux 2.6.
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.SH CONFORMING TO
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This system call is Linux specific.
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.SH NOTES
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There is no glibc wrapper for this system call,
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because it is intended for use only by the kernel and
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should never be called by applications.
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From user space, the operation of
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.BR restart_syscall (2)
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is largely invisible:
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to the process that made the system call that is restarted,
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it appears as though that system call executed and
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returned in the usual fashion.
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.\"
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.\" FIXME
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.\" There is one oddness in the implementation though, with respect to
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.\" nanosleep() (and probably also clock_nanosleep()). The scenario
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.\" is as follows:
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.\" 1. Start a nanosleep() for (say) 30 seconds,
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.\" 2. Stop the process with (say) SIGTSTP (^Z).
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.\" 3. Resume the process with SIGCONT,
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.\" 4. Upon return, the 'rem' argument of nanosleep() will contain the
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.\" remaining unslept time **at the time when SIGTSTP was delivered**.
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.\" The behavior at point 4 is odd, but doesn't violate the standards, which
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.\" specify the treatment of 'rem' only when the system call returns with
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.\" the error EINTR (i.e., the call was interrupted by a signal handler).
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.\"
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.SH SEE ALSO
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.BR sigreturn (2),
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.BR sigaction (2),
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.BR signal (7)
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.\" FIXME select(2) should probably get the restart_syscall() treatment:
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.\" If a select() call is suspended by stop-sig+SIGCONT, the time
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.\" spent suspended is *not* deducted when the select() is restarted.
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.\" FIXME: check whether recvmmsg() handles stop-sig+SIGCONT properly.
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