How do I unit test for EXC_BAD_ACCESS?

Question

I know how to resolve EXC_BAD_ACCESS issues, but I'm not sure how to unit test for it. Is there a way to capture EXC_BAD_ACCESS in code instead of simply crashing?

Here's why I ask: I have written a library that heavily uses blocks, like this:

- (void)doSomething:(void (^)())myBlock;

In my implementation of doSomething: I'm going to eventually run the block, like this:

myBlock();

If a caller passes nil for the block, then it will crash with EXC_BAD_ACCESS, so the solution is to check that the block exists, like this:

if (myBlock) {
    myBlock();
}

This nil check is pretty easy to forget, so I'd like a way to write a unit test that fails when the crash occurs. I suppose a crash could be considered a test failure, but I think it would be nicer for others trying to run the tests to see a nice failure message rather than a crash. Any ideas?

Answer 1

I think you'll need to run the test in a subprocess; then you can let the subprocess crash, check for that crash, and fail the test neatly if it occurs.

Working from Peter Hosey's singleton test code.

- (void) runTestInSubprocess:(SEL)testCmd {
        pid_t pid = fork();
        // The return value of fork is 0 in the child process, and it is
        // the id of the child process in the parent process.
        if (pid == 0) {
            // Child process: run test
            // isInSubprocess is an ivar of your test case class
            isInSubprocess = YES;
            [self performSelector:testCmd];
            exit(0);
        } else {
            // Parent process: wait for child process to end, check 
            // its status
            int status;
            waitpid(pid, &status, /*options*/ 0);
            // This was a crash; fail the test
            STAssertFalse(WIFSIGNALED(status), @"Test %@ crashed due to signal %d", NSStringFromSelector(testCmd), WTERMSIG(status));
        }
}

Each test will then run itself in a subprocess like so:

- (void) testSomething {
    if (!isInSubprocess) {
            // Hand off this test's selector to be run in a subprocess
            [self runTestInSubprocess:_cmd];
            return;
    }

    // Put actual test code here
    STAssertEquals(1, 1, @"Something wrong with the universe.");

}

You may need to tweak this; I haven't tested it.

Answer 2

I would suggest using one of the assertion macros found in the Assertions and Logging Programming Guide

So you could do something like:

NSAssert(myBlock != nil, @"myBlock must not be nil")

This enforces the preconditions that must be met before the method continues executing. It also allows the app to crash and will give you a reason why other than EXEC_BAD_ACCESS.