CocoaDev

General/WebKit is not thread-safe. At least, as far as anyone knows.

The following code, when launched from for (i=0; i<5; i++) in separated threads indexed with i, fails with SIGBUS. :

General/WebView *view = General/[[WebView alloc] init];
[view stringByEvaluatingJavaScriptFromString:@"document.location.href='http://images.google.com/images?hl=en&lr=&ie=UTF-8&q=6&btnG=Search'"];
[view setResourceLoadDelegate:self];
unsigned j=1000000;
while ((j--))
{
	General/[[NSRunLoop currentRunLoop] runUntilDate:General/[NSCalendarDate distantPast]];
}
[view release];

If something is thread-safe, then you can access it from multiple threads without worrying about simultaneous access.

If something is not thread-safe, then you can’t access it from multiple threads without explicit locking. In a context like Cocoa, where there is so much going on behind the scenes, it often means that you can’t access it from any thread besides the main thread, because it’s impossible to lock everything that a complex object accesses when it doesn’t even tell you what it accesses.

Unless General/WebView is explicitly documented somewhere as being thread-safe, then this code is wrong and dangerous. Without that explicit documentation, you must assume that it’s only safe to access General/WebViews from the main thread.

The code cannot be cleaned up unless the threads are eliminated, and the only thing to understand about using General/WebKit with threads is, as far as I know, “you can’t.”

Also, be aware that “thread-safe” is often (or at least, used to be) used in Apple’s documentation to mean “can be accessed from a worker thread, not just the main thread”. As far as I know, no Cocoa classes are actually thread-safe by the above definition except the General/NSLock family. The reason was simple: locks are always necessary to achieve thread-safety, but cost significantly when thread-safety is not required; hence, locking must be done manually.

* I think this is not correct. See http://developer.apple.com/documentation/Cocoa/Conceptual/Multithreading/articles/General/CocoaSafety.html for details.

Example quote: “The classes and functions in the following table are generally considered to be thread-safe. You can use them from multiple threads without first acquiring a lock.” *

Ah, silly me. Immutable things generally don’t need locks, do they :) As for the others, some make immediate sense, and as for General/NSLog – hurrah! Non-thread-safe error messages are a pain.