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RESTART_SYSCALL(2) Linux Programmer's Manual RESTART_SYSCALL(2)

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  sys-
     tem 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  sys-
     tem 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 pse-
     lect(2).

     From  user space, the operation of restart_syscall() is largely invisi-
     ble: 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.