Creating a simple Hello World tool that runs as a daemon - macos

Question

I built the Command Line tool (Foundation) template in Xcode. It just logs "Hello World" to the console.There is only one class in it main.m. Here's the code:
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
#autoreleasepool {
// insert code here...
NSLog(#"Hello, World!");
}
return 0;
}
Now I want to run it as a daemon and log "Hello World" to the console every 10 seconds. So I moved the product/binary to /tmp on my Mac. I created the following plist for launchd:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>helloDaemon</string>
<key>ProgramArguments</key>
<array>
<string>/tmp/helloDaemon</string>
</array>
<key>StartInterval</key>
<integer>10</integer>
</dict>
</plist>
I loaded the plist using launchctl, but I do not see any "Hello World"s in the console. Instead, I get this:
11/03/2012 00:55:35.141 com.apple.launchd: (helloDaemon) Throttling respawn: Will start in 1 seconds
11/03/2012 00:55:45.141 com.apple.launchd: (helloDaemon) Throttling respawn: Will start in 2 seconds
11/03/2012 00:55:55.140 com.apple.launchd: (helloDaemon) Throttling respawn: Will start in 3 seconds
So what's going wrong?

Answer 1

Just add to your launchd plist
<key>StandardOutPath</key>
<string>/yourpath/sample.log</string>
You can tail -f it afterwards.
More info is here:
http://developer.apple.com/library/mac/technotes/tn2083/_index.html#//apple_ref/doc/uid/DTS10003794-CH1-SUBSECTION39

Answer 2

NSLog is not working because when you launch a demon process it does not have any standard io sockets or file handles attached to it. They have to be specifically allocated. A good resource for how to create a proper daemon and how to write the console and syslog is provided in the book "Advanced Mac OS X Programming (chapter 20) by Dalrymple & Hillegass.
They have define a skeleton program that addresses the io issues you highlight. I remembered reading it a while ago and thought that maybe someday I'd need it. The authors show a sample using the syslog.h lib using openlog() and syslog() for simple communications. They also show some other lower level methods for communication to files and even sockets (for servers, etc).
I always appreciate when someone can tell me how to do something rather than reference something, but in this case, that is the best I can do. good luck.

Answer 3

With the following keys in the plist file I get most of the logs. But sometimes I am confused if I get all the NSLog output in the log file. To me it seems sometime some logs line are missing.
I am not sure if all the NSLog output of a LaunchDaemon is logged or some are skipped due to system priority.
<key>StandardOutPath</key>
<string>/var/log/mydaemon.log</string>
<key>StandardErrorPath</key>
<string>/var/log/mydaemon.log</string>