[squeak-dev] Help needed with select() socket function and signal handlers in squeakVM

David T. Lewis lewis at mail.msen.com
Mon Jan 26 01:50:49 UTC 2009

On Sun, Jan 25, 2009 at 08:50:35PM -0200, Mariano Martinez Peck wrote:
> Now, I have a problem with select() invocation. This function returns -1.
> And the printf of the errno (where select() stores the errors), says: "Error
> with select function: Interrupted system call" which means I am getting a
> EINTR error. If you see console, you can see something like this:
> timeval tv_usec value: 0
> timeval tv_sec value: 3
> timeval tv_usec value: 0
> timeval tv_sec value: 2
> timeval tv_usec value: 996000
> timeval tv_sec value: 3
> timeval tv_usec value: 0
> Error with select function: Interrupted system call
> I asked Norbert (openDBX author) and he tell me exactly this:
> "An interrupted system call is something very normal because signals can be
> sent
> at every time to the application, either by the system or by the user. I
> guess, in the Squeak code there is some signal handler installed, which does
> something if one of the defined signals arrive.
> What I will do is to hide the consequences (returning an error) by looking
> at
> the error value and reenter the select call if the system call is
> interrupted."

Norbert is correct, and his solution of reentering the system call sounds
like the right thing to do.

> All I know is squeakVM is written it SLANG and then that's transformed to C.
> So, squeakVM runs in C. So, there is where it can be a signal handler ? Is
> there some way to change that from squeak (image) ?

Any interrupt handler will service interrupts for the entire VM, so this is
not something that you would want to set up for a specific plugin. If one
plugin sets a signal handler, it affects all other plugins as well as the
VM itself.

It is actually quite easy to experiment with setting signal handlers if
you are using a Unix VM and if you have OSProcess installed. For example,
if you want to handle the SIGQUIT signal, you can evualate the expression
'OSProcess accessor forwardSigQuit' to get a Smalltalk semaphore that will be
signalled each time the operating system sends a SIGQUIT (signal number 3)
to the VM. You can have a Squeak process that waits on the semaphore and does
whatever you like.

To be clear however, Norbert's explanation is correct regardless of how
the VM chooses to handle a signal. When a system call such as select is
interrupted, the application (your plugin) needs to check for this and
retry the system call.



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