On-demand Objective-C message tracing

array.m

#import <Foundation/Foundation.h>

// clang -g -fobjc-arc -framework Foundation -o array array.m

void observeObject (id objectToWatch) {
    // Just a stub to give DTrace something to hook in to.
}

int main (void) {
    @autoreleasepool {
        NSArray *thing1 = @[ @1, @2, @3, @4];

        observeObject (thing1);
        [thing1 enumerateObjectsUsingBlock: ^(id thing, NSUInteger index, BOOL *whoa) {
                NSLog (@"Saw %@", thing);
            }];

        observeObject (nil);

        NSLog (@"it's got %ld elements", thing1.count);
    }
    return 0;

} // main

nilsends-2.d

pid$target::objc_msgSend:entry
/arg0 == 0/
{
    @stacks[ustack(5)] = count();
}

END
{
    trunc (@stacks, 20);
}

nilsends.d

pid$target::objc_msgSend:entry
/arg0 == 0/
{
    ustack ();
}

watch-array.d

/* run with
 * # dtrace -qs watch-array.d -c ./array
 */

int64_t observedObject;

pid$target::observeObject:entry
{
    printf ("observing object: %p\n", arg0);
    observedObject = arg0;
}

objc$target:::entry
/arg0 == observedObject/
{
    printf("%s %s\n", probemod, probefunc);
    /* uncomment to print stack:
     * ustack();
     */
}