restart_syscall — restart a system call after interruption by a stop signal
|There is no glibc wrapper for this system call; see NOTES.|
call is used to restart certain system calls after a process
that was stopped by a signal (e.g.,
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
Linux 2.6.22) and
FUTEX_WAIT_BITSET (since Linux 2.6.31)
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
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
mechanism built in, then, after the process resumes
execution, the time that the process spent in the stop state
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.
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From: Linus Torvalds <torvalds <at> transmeta.com>
Subject: Re: [PATCH] compatibility syscall layer (lets try again)
Date: 2002-12-05 02:51:12 GMT
See also Section 11.3.3 of Understanding the Linux Kernel, 3rd edition