dispatch_once is not reentrant

dispatch_once_reentrancy_test.m

#import <Foundation/Foundation.h>

static void test(void);

int main(int argc, char **argv) {
	@autoreleasepool {
		test();
		NSLog(@"Test succeeded");
	}
	return EXIT_SUCCESS;
}

static void test(void) {
	static unsigned count = 0;
	NSLog(@"Attempting test (%u)...", ++count);
	static dispatch_once_t token;
	dispatch_once(&token, ^{
		test();
	});
	--count;
}

Makefile

CC=clang
CFLAGS+=-std=c99 -g
LDFLAGS+=-framework Foundation 

dispatch_once_reentrancy_test: dispatch_once_reentrancy_test.o