On Mac OS X: Block until the network stack is ready. Useful for daemons which need the network to do anything useful.

waitForNetwork.m

#import <Foundation/Foundation.h>
#import <SystemConfiguration/SystemConfiguration.h>

// Wait for the network to be configured. Equivalent to `ipconfig waitall`. Adapted from ipconfig/client.c
static void _doneWaitingForNetwork(SCDynamicStoreRef session, CFArrayRef changes, void * arg){
	CFRunLoopStop(CFRunLoopGetCurrent());
}

#define STARTUP_KEY CFSTR("Plugin:IPConfiguration")
#define NETWORK_RUN_LOOP_MODE CFSTR("WaitingForNetwork")
void waitForNetwork(){
	SCDynamicStoreRef session = SCDynamicStoreCreate(NULL, (CFStringRef)[[NSProcessInfo processInfo] processName], _doneWaitingForNetwork, NULL);
	if (session == NULL) {
		asl_log(NULL, NULL, ASL_LEVEL_ERR, "SCDynamicStoreCreate failed while checking for network: %s", SCErrorString(SCError()));
		exit(EXIT_FAILURE);
	}
	CFMutableArrayRef keys = CFArrayCreateMutable(NULL, 0, &kCFTypeArrayCallBacks);
	CFArrayAppendValue(keys, STARTUP_KEY);
	SCDynamicStoreSetNotificationKeys(session, keys, NULL);
	CFRelease(keys);
	
	CFRunLoopSourceRef rls = SCDynamicStoreCreateRunLoopSource(NULL, session, 0);
	CFRunLoopAddSource(CFRunLoopGetCurrent(), rls, NETWORK_RUN_LOOP_MODE);
	
	CFPropertyListRef value = SCDynamicStoreCopyValue(session, STARTUP_KEY);
	if (value == NULL) {
		CFRunLoopRunInMode(NETWORK_RUN_LOOP_MODE, 90, false);
	} else {
		CFRelease(value);
	}
	CFRunLoopRemoveSource(CFRunLoopGetCurrent(), rls, NETWORK_RUN_LOOP_MODE);
	CFRelease(rls);
	CFRelease(session);
}