restart_syscall - restart a system call after interruption by a stop signal
long restart_syscall(void);
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
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.
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 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.
sigaction(2),
sigreturn(2),
signal(7)