Sending UDP packets in Objective C: packet arriving without any data - objective-c

Question

I'm trying to send UDP packets in objective C. More specifically, building with xcode targeting the iphone 6.1 simulator.
I can't seem to actually receive the data I send. Weirdly, I do get a data event... the data's just been truncated to length 0.
I've cut it down as much as I can, to make a dead simple test I think should pass.
#import "UdpSocketTest.h"
#include <arpa/inet.h>
#import <CoreFoundation/CFSocket.h>
#include <sys/socket.h>
#include <netinet/in.h>
#implementation UdpSocketTest
static int receivedByteCount = 0;
void onReceive(CFSocketRef socket, CFSocketCallBackType type, CFDataRef address, const void *data, void *info);
void onReceive(CFSocketRef socket, CFSocketCallBackType type, CFDataRef address, const void *data, void *info) {
// this gets called once, but CFDataGetLength(data) == 0
receivedByteCount += CFDataGetLength(data);
}
-(void) testUdpSocket {
struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_len = sizeof(addr);
addr.sin_family = AF_INET;
addr.sin_port = htons(5000); // <-- doesn't really matter, not sending from receiver
inet_pton(AF_INET, "127.0.0.1", &addr.sin_addr);
CFSocketContext socketContext = { 0, (__bridge void*)self, CFRetain, CFRelease, NULL };
// prepare receiver
CFSocketRef receiver = CFSocketCreate(kCFAllocatorDefault, PF_INET, SOCK_DGRAM, IPPROTO_UDP ,kCFSocketDataCallBack, (CFSocketCallBack)onReceive, &socketContext);
CFRunLoopAddSource(CFRunLoopGetCurrent(), CFSocketCreateRunLoopSource(NULL, receiver, 0), kCFRunLoopCommonModes);
CFSocketConnectToAddress(receiver, CFDataCreate(NULL, (unsigned char *)&addr, sizeof(addr)), -1);
// point sender at receiver
CFSocketRef sender = CFSocketCreate(kCFAllocatorDefault, PF_INET, SOCK_DGRAM, IPPROTO_UDP, kCFSocketDataCallBack, (CFSocketCallBack)onReceive, &socketContext);
CFRunLoopAddSource(CFRunLoopGetCurrent(), CFSocketCreateRunLoopSource(NULL, sender, 0), kCFRunLoopCommonModes);
CFSocketConnectToAddress(sender, CFSocketCopyAddress(receiver), -1);
// send data of sixty zeroes, allow processing to occur
CFSocketSendData(sender, NULL, (__bridge CFDataRef)[NSMutableData dataWithLength:60], 2.0);
[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1.0]];
// did the data arrive?
STAssertTrue(receivedByteCount > 0, #"");
// nope
}
#end
What am I doing wrong? I know it's something obvious, but I can't see it.

Answer 1

I'm surprised you're getting any callbacks at all—you're never actually binding your receiver to your local port. You're creating two sockets and telling both of them "I went to send data to 127.0.0.1:5000", but nowhere are you saying "I want to receive data on 127.0.0.1:5000.
In order to do that, you should be calling CFSocketSetAddress on the receiver socket, not CFSocketConnectToAddress. This is equivalent to calling the bind(2) system call on the underlying native BSD socket.