Put a mac's AirPort in HostAP mode. Updated to work with OS X 10.8.

hostap.m

#import <CoreWLAN/CoreWLAN.h>
#import <objc/message.h>

int main(int argc, char* argv[]) {
  @autoreleasepool {
    int ch;
    NSString *ssid = nil, *password = nil;

    while((ch = getopt(argc, argv, "s:p:h")) != -1) {
      switch(ch) {
      case 's':
        ssid = [NSString stringWithUTF8String:optarg];
        break;
      case 'p':
        password = [NSString stringWithUTF8String:optarg];
        break;
      case '?':
      case 'h':
      default:
        printf("USAGE: %s [-s ssid] [-p password] [-h] command\n", argv[0]);
        printf("\nOPTIONS:\n");
        printf("   -s ssid     SSID\n");
        printf("   -p password WEP password\n");
        printf("   -h          Print help\n");
        printf("\nCOMMAND:\n");
        printf("   status      Print interface mode\n");
        printf("   start       Start Host AP mode\n");
        printf("   stop        Stop Host AP mode\n");
        return 0;
      }
    }

    NSString *command = nil;
    if(argv[optind]) {
      command = [NSString stringWithUTF8String:argv[optind]];
    }

    CWInterface *iface = [CWInterface interface];

    if(!command || [command isEqualToString:@"status"]) {
      NSString *mode = nil;
      switch(iface.interfaceMode) {
      case kCWInterfaceModeStation:
        mode = @"Station";
        break;
      case kCWInterfaceModeIBSS:
        mode = @"IBSS";
        break;
      case kCWInterfaceModeHostAP:
        mode = @"HostAP";
        break;
      case kCWInterfaceModeNone:
      default:
        mode = @"None";
      }
      printf("%s\n", [mode UTF8String]);
    } else if([command isEqualToString:@"stop"]) {
      // Stop Host AP mode
      if(getuid() != 0) {
        printf("this may need root (trying anyway)...\n");
      }
      objc_msgSend(iface, @selector(stopHostAPMode));
    } else if([command isEqualToString:@"start"]) {
      if(!ssid) {
        printf("error: an ssid must be specified\n");
        return 1;
      }

      // known security types:
      //   2: no securiry
      //   16: wep
      // Note: values [-127..127] have been tried, and all but these return errors.
      int securityType = 2;
      if(password) {
        if([password length] < 10) {
          printf("error: password too short (must be >= 10 characters)\n");
          return 1;
        }
        securityType = 16;
      }

      NSSet *chans = [iface supportedWLANChannels];
      //printf("chan count: %lu\n", [chans count]);

      NSEnumerator *enumerator = [chans objectEnumerator];
      CWChannel *channel;
      while ((channel = [enumerator nextObject])) {
        //printf("channel: %lu\n", [channel channelNumber]);
        if ([channel channelNumber] == 9)
          break;
      }

      // Start Host AP mode
      NSError *error = nil;
      objc_msgSend(iface,
                   //@selector(startIBSSModeWithSSID:security:channel:password:error:),
                   @selector(startHostAPModeWithSSID:securityType:channel:password:error:),
                   //[@"hunter2" dataUsingEncoding:NSUTF8StringEncoding],
                   [ssid dataUsingEncoding:NSUTF8StringEncoding],
                   securityType,
                   channel,
                   //@"abc123ffff",
                   password,
                   &error);
      if(error) {
        printf("startHostAPModeWithSSID error: %s\n", [error.localizedDescription UTF8String]);
        return 1;
      }
    }

    return 0;
  }
}

Makefile

all: hostap

hostap: hostap.m
	clang -framework Foundation -framework CoreWLAN -o $@ $<

clean:
	rm -rf hostap

MIT-LICENSE.txt

Copyright (C) 2012 Yoshimasa Niwa

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Have you tried this with 10.9 Mavericks? Doesn't seem to be working.
$ sudo ./hostap status
None
$ sudo ./hostap -s testssid start
startHostAPModeWithSSID error: Operation not permitted

It works in 10.10 for a couple of seconds. Drops connection then. It would be really appreciated if someone experienced can troubleshoot this because there is no option to create a secured WiFi network in Yosemite.

Here is a solution that works for 10.10: http://stackoverflow.com/questions/31342348/programmatically-create-ad-hoc-network-os-x

That's not a "solution" for 10.10, that creates an ad-hoc (IBSS) network with its huge limitations. This script, in contrast, used to create a full access point.

This seems to work partially on 10.14. Running sudo ./hostap -s MyNetworkName start successfully enters HostAP mode and creates the access point (you can see the Wi-Fi icon in the menu bar turn to the one with the up arrow). I can see the access point exists in the Wi-Fi menus of my iOS and Android devices, but when I attempt to join the on any device, the device stalls and does not join the network (on Android the Wi-Fi listing says "obtaining IP address" continuously and on iOS the Wi-Fi listing keeps showing the loading symbol). Is there any way around this issue?

Another error I run into: if I try to start HostAP mode with a password sudo ./hostap -s MyNetworkName -p 12345abcde start, I get the error:
startHostAPModeWithSSID error: The operation couldn’t be completed. (com.apple.wifi.apple80211API.error error -3900.)

Has anyone come across any solution to enter HostAP mode programmatically?