HOMEWORK 3 (COMMENTS AND COMMON MISTAKES)

<< General >>
 
o If a problem asks "why is the output correct", the answer is not
	"because it works as expected".

	+ What are the properties that make you think the output is
		correct?  Are the times consistent with past experience?
		e.g.,	The output of 'ls' ...
	+ Try to quantify instead of using "very large" and "very very large"

<< Problem 5>>

<< Problem 6>>

o Problems with using gettimeofday(3)

  + gettimeofday(3) returns time since Jan 1, 1970.
  
  	- The tv_sec and tv_usec components are each 32-bit integers
		(long ints)
  	- This will result in the number of seconds being a 10-digit
		decimal number after conversion and the number of
		microseconds being a 16-digit decimal number.
	- int64_t converts to a 20-digit decimal number.
	- Need to be careful when computing the difference (tend - tbegin).

  + Need to use BOTH components of a struct timeval and treat as a
  	vector

	Example:
			sec  usec
			  |  |
			  v  v
		tbegin = (X, Y)
		tend   = (X+1, Y-Z)

	If you subtract component-by-component, this will happen:

		tend - tbegin = (1, -Z)

	i.e., elapsed time is 1 sec, -Z usec.  But you really wanted:

		tend - tbegin = (0, 10^6-Z)

  + Truncation can occur unless you force the use of 64-bit integer
  	arithmetic

	int64_t	t = 1000000*(end.tv_sec - begin.tv_sec)
				+ (end.tv_usec - begin.tv_usec);

	is likely to compute using int32_t and then truncate during
	assignment.  Better is something like this which forces use
	of 64-bit integer arithmetic:

	int64_t	t = 1000000*(((int64_t)end.tv_sec) - begin.tv_sec)
				+ (end.tv_usec - begin.tv_usec);

	double also works.

  + Would help if the key time output values were labeled with units.
  	Maybe ok to leave off units in debug output.

<< Problem 7>>

o There are various command line flags for the ps command

	ps -e		Show every process
	ps -ejH		Show process tree
	ps l		Show BSD long format