man2/restart_syscall.2

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  25 .\" http://thread.gmane.org/gmane.linux.kernel/76552/focus=76803
  26 .\" From: Linus Torvalds <torvalds <at> transmeta.com>
  27 .\" Subject: Re: [PATCH] compatibility syscall layer (lets try again)
  28 .\" Newsgroups: gmane.linux.kernel
  29 .\" Date: 2002-12-05 02:51:12 GMT
  30 .\"
  31 .\" See also Section 11.3.3 of Understanding the Linux Kernel, 3rd edition
  32 .\"
  33 .TH RESTART_SYSCALL 2 2016-10-08 "Linux" "Linux Programmer's Manual"
  34 .SH NAME
  35 restart_syscall \- restart a system call after interruption by a stop signal
  36 .SH SYNOPSIS
  37 .B int restart_syscall(void);
  38 .PP
  39 .IR Note :
  40 There is no glibc wrapper for this system call; see NOTES.
  41 .SH DESCRIPTION
  42 The
  43 .BR restart_syscall ()
  44 system call is used to restart certain system calls
  45 after a process that was stopped by a signal (e.g.,
  46 .BR SIGSTOP
  47 or
  48 .BR SIGTSTP )
  49 is later resumed after receiving a
  50 .BR SIGCONT
  51 signal.
  52 This system call is designed only for internal use by the kernel.
  53 .PP
  54 .BR restart_syscall ()
  55 is used for restarting only those system calls that,
  56 when restarted, should adjust their time-related parameters\(emnamely
  57 .BR poll (2)
  58 (since Linux 2.6.24),
  59 .BR nanosleep (2)
  60 (since Linux 2.6),
  61 .BR clock_nanosleep (2)
  62 (since Linux 2.6),
  63 and
  64 .BR futex (2),
  65 when employed with the
  66 .BR FUTEX_WAIT
  67 (since Linux 2.6.22)
  68 and
  69 .BR FUTEX_WAIT_BITSET
  70 (since Linux 2.6.31)
  71 operations.
  72 .\" These system calls correspond to the special internal errno value
  73 .\" ERESTART_RESTARTBLOCK. Each of the system calls has a "restart"
  74 .\" helper function that is invoked by restart_syscall().
  75 .\" Notable (as at Linux 3.17) is that poll() has such a "restart"
  76 .\" function, but ppoll(), select(), and pselect() do not.
  77 .\" This means that the latter system calls do not take account of the
  78 .\" time spent in the stopped state when restarting.
  79 .BR restart_syscall ()
  80 restarts the interrupted system call with a
  81 time argument that is suitably adjusted to account for the
  82 time that has already elapsed (including the time where the process
  83 was stopped by a signal).
  84 Without the
  85 .BR restart_syscall ()
  86 mechanism, restarting these system calls would not correctly deduct the
  87 already elapsed time when the process continued execution.
  88 .SH RETURN VALUE
  89 The return value of
  90 .BR restart_syscall ()
  91 is the return value of whatever system call is being restarted.
  92 .SH ERRORS
  93 .I errno
  94 is set as per the errors for whatever system call is being restarted by
  95 .BR restart_syscall ().
  96 .SH VERSIONS
  97 The
  98 .BR restart_syscall ()
  99 system call is present since Linux 2.6.
 100 .SH CONFORMING TO
 101 This system call is Linux-specific.
 102 .SH NOTES
 103 There is no glibc wrapper for this system call,
 104 because it is intended for use only by the kernel and
 105 should never be called by applications.
 106 .PP
 107 The kernel uses
 108 .BR restart_syscall ()
 109 to ensure that when a system call is restarted
 110 after a process has been stopped by a signal and then resumed by
 111 .BR SIGCONT ,
 112 then the time that the process spent in the stopped state is counted
 113 against the timeout interval specified in the original system call.
 114 In the case of system calls that take a timeout argument and
 115 automatically restart after a stop signal plus
 116 .BR SIGCONT ,
 117 but which do not have the
 118 .BR restart_syscall ()
 119 mechanism built in, then, after the process resumes execution,
 120 the time that the process spent in the stop state is
 121 .I not
 122 counted against the timeout value.
 123 Notable examples of system calls that suffer this problem are
 124 .BR ppoll (2),
 125 .BR select (2),
 126 and
 127 .BR pselect (2).
 128 .PP
 129 From user space, the operation of
 130 .BR restart_syscall ()
 131 is largely invisible:
 132 to the process that made the system call that is restarted,
 133 it appears as though that system call executed and
 134 returned in the usual fashion.
 135 .SH SEE ALSO
 136 .BR sigaction (2),
 137 .BR sigreturn (2),
 138 .BR signal (7)
 139 .\" FIXME . ppoll(2), select(2), and pselect(2)
 140 .\"     should probably get the restart_syscall() treatment:
 141 .\"     If a select() call is suspended by stop-sig+SIGCONT, the time
 142 .\"     spent suspended is *not* deducted when the select() is restarted.
 143 .\" FIXME . check whether recvmmsg() handles stop-sig+SIGCONT properly.