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

i'm trying send udp packets in objective c. more specifically, building xcode targeting iphone 6.1 simulator.

i can't seem receive data send. weirdly, data event... data's been truncated length 0.

i've cut down as can, make dead simple test 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) {     // gets called once, 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 matter, not sending 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 @ 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 occur     cfsocketsenddata(sender, null, (__bridge cfdataref)[nsmutabledata datawithlength:60], 2.0);     [[nsrunloop currentrunloop] rununtildate:[nsdate datewithtimeintervalsincenow:1.0]];      // did data arrive?     stasserttrue(receivedbytecount > 0, @"");     // nope } @end 

what doing wrong? know it's obvious, can't see it.

i'm surprised you're getting callbacks @ all—you're never binding receiver local port. you're creating 2 sockets , telling both of them "i went send data 127.0.0.1:5000", saying "i want receive data on 127.0.0.1:5000.

in order that, should calling cfsocketsetaddress on receiver socket, not cfsocketconnecttoaddress. equivalent calling bind(2) system call on underlying native bsd socket.