Re: [webkit-dev] [JavaScriptCore] When to JSValueProtect?
On Dec 7, 2013, at 12:15 PM, Alexei Sholik alcosho...@gmail.com wrote: The garbage collector scans the C stack. Hm, let me make sure I understand this correctly. ... I'm asking because I've never previously dealt with a library that scans the host application's stack. So it sounds pretty incredible to me. Every JSC vm will do a full conservative scan of the in use portion of the stack for every thread in your application whenever it needs to GC. Anything that looks like it could be a pointer to a gc value is treated as though it is _definitely_ a GC value and marked accordingly. Thanks! On Sat, Dec 7, 2013 at 9:37 PM, Geoffrey Garen gga...@apple.com wrote: At this point, the code is not inside the JS stack, so is it possible for an object to be collected between the calls to JSObjectMake and JSObjectCallAsFunction? The garbage collector scans the C stack. Geoff -- Best regards Alexei Sholik ___ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev ___ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev
Re: [webkit-dev] [JavaScriptCore] When to JSValueProtect?
Hi Geoffrey, Therefore, if I'm not immediately returning the created object, it might get cleaned up prematurely. Because the GC scans the stack, and the value you just created is on the stack, it won’t be garbage collected right away. What about the case when the creation of an object originates from C++ land? For example, if I tap on a view and it needs to invoke a JS callback. I'm creating an object to pass to JSEvaluateScript or as an argument to JSObjectCallAsFunction. At this point, the code is not inside the JS stack, so is it possible for an object to be collected between the calls to JSObjectMake and JSObjectCallAsFunction? Thanks! On Sat, Dec 7, 2013 at 8:55 PM, Geoffrey Garen gga...@apple.com wrote: My question in short: is it necessary to call JSValueProtect right after object/value creation in C++? No. I don't know the semantics of the GC, but I'm assuming that almost any call to a JSC function that takes a context may run a garbage collection cycle. Yes. Therefore, if I'm not immediately returning the created object, it might get cleaned up prematurely. Because the GC scans the stack, and the value you just created is on the stack, it won’t be garbage collected right away. Could you share some advice on the proper usage of JSValueProtect and on keeping objects from going away unexpectedly? If you store a JSValueRef or JSObjectRef into a C/C++ heap object, then you should JSValueProtect the JSValueRef/JSObjectRef, since your reference to it will outlast the pointer to it on the stack. Use JSValueProtect/JSValueUnprotect just like you would use any other reference-counting API — such as CFRetain/CFRelease, NSObject retain/release, or IUnknown AddRef/RemoveRef — with the added feature that you do not need to retain local variables. Geoff -- Best regards Alexei Sholik ___ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev
Re: [webkit-dev] [JavaScriptCore] When to JSValueProtect?
At this point, the code is not inside the JS stack, so is it possible for an object to be collected between the calls to JSObjectMake and JSObjectCallAsFunction? The garbage collector scans the C stack. Geoff ___ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev
Re: [webkit-dev] [JavaScriptCore] When to JSValueProtect?
My question in short: is it necessary to call JSValueProtect right after object/value creation in C++? No. I don't know the semantics of the GC, but I'm assuming that almost any call to a JSC function that takes a context may run a garbage collection cycle. Yes. Therefore, if I'm not immediately returning the created object, it might get cleaned up prematurely. Because the GC scans the stack, and the value you just created is on the stack, it won’t be garbage collected right away. Could you share some advice on the proper usage of JSValueProtect and on keeping objects from going away unexpectedly? If you store a JSValueRef or JSObjectRef into a C/C++ heap object, then you should JSValueProtect the JSValueRef/JSObjectRef, since your reference to it will outlast the pointer to it on the stack. Use JSValueProtect/JSValueUnprotect just like you would use any other reference-counting API — such as CFRetain/CFRelease, NSObject retain/release, or IUnknown AddRef/RemoveRef — with the added feature that you do not need to retain local variables. Geoff ___ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev
Re: [webkit-dev] [JavaScriptCore] When to JSValueProtect?
Will JSC scan the current stack which arg1 and arg2 are on? Yes. Will it scan the stack even further back? Yes. Geoff ___ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev
[webkit-dev] [JavaScriptCore] When to JSValueProtect?
Hi, My question in short: is it necessary to call JSValueProtect right after object/value creation in C++? I don't know the semantics of the GC, but I'm assuming that almost any call to a JSC function that takes a context may run a garbage collection cycle. Therefore, if I'm not immediately returning the created object, it might get cleaned up prematurely. Could you share some advice on the proper usage of JSValueProtect and on keeping objects from going away unexpectedly? Some background info: I'm experiencing intermittent bugs in my C++ to JS binding, I'm only guessing that GC might be related to that. What I'm seeing is that objects created in C++ are occasionally missing their properties. In one example, there is a single code path from C++ to JS that takes a native object and creates a corresponding JS object using appropriate JSClassRef (which is prepopulated with some methods). In the JS land, there is always the same method called on the object (the one that has just been created in C++ land with the appropriate JSClassRef). 9 out of 10 times it works fine, but every once in a while I get an exception undefined is not a function, i.e. the method on the object is missing. The way I'm linking native and JS objects is by keeping an associative array of their pointers, so that I can get back to the native object in the C++ callback called from JS. -- Best regards Alexei Sholik ___ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev
