Time in Linux programming

Kernel measures the passage of the time in three different ways:

1. Wall time ---> measuring absolute time
   
2. Process time
   
3. Monotonic time ---> calculating relative time
   
   Unix systems represent absolute time as the number of elapsed seconds since the epoch, which is defined as 00:00:00 UTC on the morning of 1 January 1970.
   SO, even absolute time is, at a low level, relative time.
   
   The following table shows some useful functions for uing time:
   

Name	second	Microsecond	Nanosecond
Getting the current time of day	time()	gettimeofday()	clock_gettime()
Setting the current time of day	stime()	settimeofday()	clock_settime()
Sleeping	sleep()	usleep()	nanosleep()

Now Microsoft, ipod nano are in my mind, are these names from here? So interesting.

With a 32-bit long type, time_t will let us have the Y2K mess all over again --- in 2038!

and "Come 22:14:07 on Monday, 18 January 2038, most systems and software will be 64-bit." said Robert Love. Will these happen, let's go and see.

POSIX Clocks have four of the linux standard time source:

• CLOCK_MONOTONIC
  
• CLOCK_PROCESS_CPUTIME_ID
  
• CLOCK_REALTIME
  
• CLOCK_THREAD_CPUTIME_ID
  

Tuning the system clock

example by a Makefile

make looks at the file modification timestamps of the source file versus the object file. If the source file is newer than the object file, make rebuilds the source file into an updated object file. If the source file is not newer than the object, however, no action is taken

basic git command

git is the fast version control system

Now I am studying how to use git

• Here are the commands for common using:
  
  

1. Use git clone to check out sources. i.e. Download pulseaudio sources:
   
   

   git clone git://git.0pointer.de/pulseaudio.git
   
   
   

2. Use git status to show your working tree status
   
   

   git status
   
   

3. Use git pull to update the codes to the newest version
   
   

   git pull origin master
   
   

4. Add or edit your file
   
   
5. Commit your new file
   
   

   git add your-file
   
   git commit -m 'log'
   
   

6. Push your change to remote repository
   
   

   git push origin master
   
   

• Here is a sample to generate a patch
  
  

  git status
  
  view /home/usr/.gitconfig 
  
  git add src/pulsecore/random.c
  
  git log -v
  
  git commit -m 'core: Fix variable "has_whined" value bug'
  
  git format-patch -s -1
  
  view 0001-core-Fix-variable-has_whined-value-bug.patch