Saturday, October 21, 2006

MacOS run loops and console mode

Run loops do NOT run automatically in console mode, not even on the main thread. It kind of makes sense, run loops are one of the mechanisms that support GUI events. So you have to create a run loop manually when running in console mode; it can use multiple timers, and it will pre-empt the main thread, whose execution will only resume after the run loop's timers finish running.

Sunday, October 15, 2006

Adventures in multithreading

An insidious race condition arises in the following situation (which I encountered in Objective-C, but any language that passes by reference will allow for the same):

I have a consumer function which writes a message to a file or database and which can be called by multiple threads - so it is LOCKed. This function is called by threads generated by a loop, where the thread instantiation function takes as parameter a string which is modified by each loop iteration. E.g., in pseudo-C:

string msg;
for( i = 0; i < 10; i++ ){
fsprintf( msg, "parameter: %i", i );
launchThread( msg );

void launchThread( string parm ){
plock lock;
fprintf( fHandle, parm );
plock unlock;

Of course since the fprintf needs to be atomic (in order not to generate a bus error), it is the one that has to be locked. However, msg is a shared resource as well. If you run this code as it is you will get an output similar to the following:


Instead of the expected:


That is because msg is modified by the loop and by the time thread #x has picked it up, who knows what value it has - certainly not one in sync with #x.

The solution is to provide as parameter to the thread a full immutable copy of msg and not a reference to msg.