Tutorial :AIO on OS X vs Linux - why it doesn't work on Mac OS X 10.6


My question is really simple. Why the code below does work on Linux, and doesn't on Mac OS X 10.6.2 Snow Leopard.

To compile save the file to aio.cc, and compile with g++ aio.cc -o aio -lrt on Linux, and g++ aio.cc -o aio on Mac OS X. I'm using Mac OS X 10.6.2 for testing on a Mac, and Linux kernel 2.6 for testing on Linux.

The failure I see on OS X is aio_write fails with -1 and sets errno to EAGAIN, which simply means "Resource temporarily unavailable". Why is that?

extern "C" {  #include <aio.h>  #include <sys/types.h>  #include <sys/socket.h>  #include <sys/types.h>  #include <arpa/inet.h>  #include <netinet/in.h>  #include <errno.h>  #include <signal.h>  }  #include <cassert>  #include <string>  #include <iostream>    using namespace std;    static void  aio_completion_handler(int signo, siginfo_t *info, void *context)  {    using namespace std;    cout << "BLAH" << endl;  }      int main()  {    int err;      struct sockaddr_in sin;    memset(&sin, 0, sizeof(sin));      sin.sin_port = htons(1234);    sin.sin_addr.s_addr = inet_addr("");    sin.sin_family = PF_INET;      int sd = ::socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);    if (sd == -1) {      assert(!"socket() failed");    }      const struct sockaddr *saddr = reinterpret_cast<const struct sockaddr *>(&sin);    err = ::connect(sd, saddr, sizeof(struct sockaddr));    if (err == -1) {      perror(NULL);      assert(!"connect() failed");    }      struct aiocb *aio = new aiocb();    memset(aio, 0, sizeof(struct aiocb));      char *buf = new char[3];    buf[0] = 'a';    buf[1] = 'b';    buf[2] = 'c';    aio->aio_fildes = sd;    aio->aio_buf = buf;    aio->aio_nbytes = 3;      aio->aio_sigevent.sigev_notify = SIGEV_SIGNAL;    aio->aio_sigevent.sigev_signo = SIGIO;    aio->aio_sigevent.sigev_value.sival_ptr = &aio;      struct sigaction sig_act;      sigemptyset(&sig_act.sa_mask);    sig_act.sa_flags = SA_SIGINFO;    sig_act.sa_sigaction = aio_completion_handler;      sigaction(SIGIO, &sig_act, NULL);      errno = 0;    int ret = aio_write(aio);    if (ret == -1) {      perror(NULL);    }      assert(ret != -1);    }  

UPDATE (Feb 2010): OSX does not support AIO on sockets at all. Bummer!


The presented code was tested on Mountain Lion 10.8.2. It works with a small correction. The line
"aio->aio_fildes = sd;"

should be changed for example to:
aio->aio_fildes = open( "/dev/null", O_RDWR);

to get the expected result.

see manual. "The aio_write() function allows the calling process to perform an asynchronous write to a previously opened file."


I have code very similar to yours on 10.6.2 (but writing to a file) working without any problems - so it is possible to do what you're trying.

Just out of curiosity, what value are you using for the SIGIO constant ? I found that an invalid value here in OS X would casue aio_write to fail - so I always pass SIGUSR1.

Maybe check the return value of sigaction() to verify the signal details?


The points raised in your links all point to a different method for raising io completion notifications (e.g. kqueue which is a BSD specific mechanism), but doesn't really answer your question re POSIX methods for async io. and whether they work on Darwin.

The UNIX world really is a mish mash of solutions for this, and it would be really good if there was one tried and tested solutiom that worked across all platforms, alas currently there's not - POSIX being the one that aims for the most consistency.

It's a bit of a stab in the dark, but it might be useful as well to set nonblocking on your socket handle ( i.e. set socket option O_NONBLOCK ) as well as using SIGUSR1

If I get some time I'll work with your socket sample and see if I can get anything out of that too.

Best of luck.


OSX Allows you to use sockets via the (CF)RunLoop. Or getting callbacks from the runloop. That is the most elegant way I have found to use async IO on mac. You can use your existing socket and do a CFSocketCreateWithNative. And register callbacks on your runloop.

Here is a small snippet of code that shows how it can be setup, incomplete since I have cut down on a source file...

// This will setup a readCallback   void SocketClass::setupCFCallback() {     CFSocketContext     context = { 0, this, NULL, NULL, NULL };    if (CFSocketRef macMulticastSocketRef = CFSocketCreateWithNative(NULL, socketHandle_, kCFSocketReadCallBack,readCallBack, &context)) {      if (CFRunLoopSourceRef macRunLoopSrc = CFSocketCreateRunLoopSource(NULL, macMulticastSocketRef, 0)) {          if (!CFRunLoopContainsSource(CFRunLoopGetCurrent(), macRunLoopSrc, kCFRunLoopDefaultMode)) {              CFRunLoopAddSource(CFRunLoopGetCurrent(), macRunLoopSrc, kCFRunLoopDefaultMode);              macRunLoopSrc_ = macRunLoopSrc;          }          else              CFRelease(macRunLoopSrc);      }      else          CFSocketInvalidate(macMulticastSocketRef);      CFRelease(macMulticastSocketRef);  }  }        void SocketClass::readCallBack(CFSocketRef inref, CFSocketCallBackType  type,CFDataRef , const void *, void *info) {  if (SocketClass*    socket_ptr = reinterpret_cast<SocketClass*>(info))      socket_ptr->receive(); // do stuff with your socket    }  

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