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.

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

COLOPHON
       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-10-08		    RESTART_SYSCALL(2)