RESTART_SYSCALL(2) Linux Programmer's Manual RESTART_SYSCALL(2)
restart_syscall - restart a system call after interruption by a stop
Note: There is no glibc wrapper for this system call; see NOTES.
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.
The return value of restart_syscall() is the return value of whatever
system call is being restarted.
errno is set as per the errors for whatever system call is being
restarted by restart_syscall().
The restart_syscall() system call is present since Linux 2.6.
This system call is Linux-specific.
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-
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
sigaction(2), sigreturn(2), signal(7)
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Linux 2017-09-15 RESTART_SYSCALL(2)
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