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

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

SYNOPSIS
       long 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 re‐
       sumed 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 FU‐
       TEX_WAIT_BITSET (since Linux 2.6.31) operations.  restart_syscall() restarts the interrupted system call with a time argument that is suitably  adjusted  to  ac‐
       count  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)

Linux                                                                          2021-03-22                                                             RESTART_SYSCALL(2)