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NAME

       restart_syscall - restart a system call after interruption by a stop signal

SYNOPSIS

       int restart_syscall(void);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

       The  restart_syscall() system call is used to restart certain system calls after a process
       that was stopped by a signal (e.g., SIGSTOP or SIGTSTP) is later resumed after receiving a
       SIGCONT signal.  This system call is designed only for internal use by the kernel.

       restart_syscall()  is  used  for  restarting only those system calls that, when restarted,
       should  adjust  their  time-related  parameters—namely  poll(2)  (since   Linux   2.6.24),
       nanosleep(2)  (since  Linux 2.6), clock_nanosleep(2) (since Linux 2.6), and futex(2), when
       employed with the FUTEX_WAIT (since  Linux  2.6.22)  and  FUTEX_WAIT_BITSET  (since  Linux
       2.6.31)  operations.   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
       restart_syscall() mechanism, restarting these system calls would not correctly deduct  the
       already elapsed time when the process continued execution.

RETURN VALUE

       The return value of restart_syscall() is the return value of whatever system call is being
       restarted.

ERRORS

       errno is  set  as  per  the  errors  for  whatever  system  call  is  being  restarted  by
       restart_syscall().

VERSIONS

       The restart_syscall() system call is present since Linux 2.6.

CONFORMING TO

       This system call is Linux-specific.

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.

       The kernel uses restart_syscall() to ensure that when a system call is restarted  after  a
       process  has  been stopped by a signal and then resumed by 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  SIGCONT,  but  which  do  not  have  the
       restart_syscall()  mechanism built in, then, after the process resumes execution, the time
       that the process spent in the stop  state  is  not  counted  against  the  timeout  value.
       Notable  examples  of  system  calls that suffer this problem are ppoll(2), select(2), and
       pselect(2).

       From user space, the operation of 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.

SEE ALSO

       sigaction(2), sigreturn(2), signal(7)

COLOPHON

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