Very Computer

I am trying to use a timer and SIGALRM to call some functions
time-dependent. I set sa_flags to SA_RESTART to automatically restart
interrupted system calls (see code below). But if my program is
blocked inside select when the timer fires, select aborts with
errno == -EINTR instead of being restarted. What ist wrong?

Thanks for any hint,

        Dirk

--------- code -----------------

struct sigaction sa_old;

void client_tmsinit(void)
{

  struct sigaction sa;

  sa.sa_handler = do_timer;
  sigemptyset(&sa.sa_mask);
  sa.sa_flags = SA_RESTART;
  sigaction(SIGALRM, &sa, &tms.sa_old);  

Quote:}

  if (tms.used_timer) {
    val.it_interval.tv_usec = 0;
    val.it_interval.tv_sec = 0;
    val.it_value.tv_usec = (expires % 1000) * 1000;
    val.it_value.tv_sec = expires/1000;
    n =  setitimer (ITIMER_REAL, &val, NULL);
    if (n<0) {
      perror("set_itimer");
    }
  }

Quote:}

Quote:>I am trying to use a timer and SIGALRM to call some functions
>time-dependent. I set sa_flags to SA_RESTART to automatically restart
>interrupted system calls (see code below). But if my program is
>blocked inside select when the timer fires, select aborts with
>errno == -EINTR instead of being restarted. What ist wrong?

--
http://www.spinics.net/linux/

Quote:>>I am trying to use a timer and SIGALRM to call some functions
>>time-dependent. I set sa_flags to SA_RESTART to automatically restart
>>interrupted system calls (see code below). But if my program is
>>blocked inside select when the timer fires, select aborts with
>>errno == -EINTR instead of being restarted. What ist wrong?

>Just test the result and reissue the select.  SA_RESTART doesn't
>handle all system calls.

Thanks
        Dirk

In order for a system call to be interrupted
it must go into interruptible sleep. When it
is waked up again it must check signals and
return an error if there has been a signal.
This error determines the behavior:

-ERESTARTNOHAND or -EINTR means that this
syscall will never be restarted.

-ERESTARTSYS means that this syscall will
be restarted if the SA_RESTART is set and
otherwise not.

-ERESTARTNOINTR means that this syscall will
always be restarted.

You could also write code that will handle
the EINTR error condition, just restart the
systemcall (and take the passed time into
account if the syscall involves a timeout
or a sleep for a specific time.)

--
Kasper Dupont

- wait(), waitpid(), wait3(), and wait4().

- read(), readv(), write(), writev(), and ioctl() when applied to "slow"
devices (e.g. terminal reads).

- Open() in cases where it can block (e.g., when opening FIFOs)

- Various sockets calls: accept(), connect(), send(), sendmsg(), recv(),
and recvmsg().

- fcntl() when used to apply file region locks

The following are not restarted:

- The poll() and select() I/O multiplexing calls.  (Under System V AFAIK,
these system calls are restarted if the SA_RESTART option is specified.)

- System V IPC: semop(), msgrcv(), and msgsnd().

This list is current and accurate for 2.2, and should be also for 2.4 (I
had a read of the 2.4 sources, but didn't do all the tests yet)

Cheers

Michael

Quote:

>>>I am trying to use a timer and SIGALRM to call some functions
>>>time-dependent. I set sa_flags to SA_RESTART to automatically restart
>>>interrupted system calls (see code below). But if my program is
>>>blocked inside select when the timer fires, select aborts with
>>>errno == -EINTR instead of being restarted. What ist wrong?

>>Just test the result and reissue the select.  SA_RESTART doesn't
>>handle all system calls.

>OK, to put it this way: how do i know for sure, which system call
>will be restarted and which one not?

- wait(), waitpid(), wait3(), and wait4().

- read(), readv(), write(), writev(), and ioctl() when applied to
"slow" devices (e.g. terminal reads).

- Open() in cases where it can block (e.g., when opening FIFOs)

- Various sockets calls: accept(), connect(), send(), sendmsg(),
recv(), and recvmsg().

- fcntl() when used to apply file region locks

The following are not restarted:

- The poll() and select() I/O multiplexing calls.  (Under System V
AFAIK, these system calls are restarted if the SA_RESTART option is
specified.)

- System V IPC: semop(), msgrcv(), and msgsnd().

This list is current and accurate for 2.2, and should be also for 2.4
(I had a read of the 2.4 sources, but didn't do all the tests yet)

Cheers

Michael