Discussion Week 5

Overview 
   Quiz
   opendir/readdir/closedir/stat
   pattern matching
   Debugging techniques
     programatic
     Debuggers
   Function pointers

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Quiz



Function Pointers

   sort example



char *names[] = { "zack", "bob", "alice" }
void bsort (char *v[],  int size)
{
   int i,j;
   for (i=0; i < size; i++)
     for (j = i+1; j < size; j++)
      if (strcmp (v[i], v[j]) > 0)
       swap (v[i], v[j]);
}

But now I have a struct

   struct person { char name[32]; int age; };

   typedef int (*comp_fct) (void *, void *);


void bsort (struct person *v[],  int size)
{
   int i,j;
   for (i=0; i < size; i++)
     for (j = i+1; j < size; j++)
      if (strcmp (v[i]->name, v[j]->name) > 0)
       swap (v[i], v[j]);
}

   Note : Need only change compare function (the rest stays
   the same.

   What about if I wanted to sort on age, --> need
   to change the compare function
   

   typedef int (*comp_fct) (void *, void *);


void bsort (struct person *v[],  int size, comp_fct compare)
{
   int i,j;
   for (i=0; i < size; i++)
     for (j = i+1; j < size; j++)
      if ((*compare) (v[i], v[j]) > 0)
       swap (v[i], v[j]);
}

int person_compare (void *v1, void *v2) 
{
    struct person *p1 = (struct person*)v1;
    struct person *p2 = (struct person*)v2;

    return strcmp (p1->name, p2->name);  

    // or

    if (p1->age < p2->age) return -1;
    if (p1->age > p2->age) return 1;
    return 0;
}

Debugging Techniques
  printf

	Simple printf ("X = %d \n");

  DEBUG macros

	#define DEBUG(X) printf X

        DEBUG(("X = %d\n"));

	Turn off debugging by simply

	#define DEBUG(X)

  levels

	Sometimes you need levels of report

	extern int dlevel;

	#define DEBUGL(L,X) do { if (dlevel > (L)) printf X ; }

	DEBUGL(10, ("low priority X=%d\n"));

	DEBUGL(1, ("high pri X=%d\n"));


  assert
	Assert something is about the state of your program.

	#include <assert.h>
	int fact (int x){
	   assert (x>=0);
	   ...

  GDB

    http://www.cs.ucsb.edu/Facilities/Software/gdb.html

    starting

	Compile all programs with -g
	gcc -g -c board.c main.o
	gcc -g -o mancala board.o main.o

	gdb mancala or run under xemacs M-x gdb

	gdb> run

    break points

	break main  ; set a break point
	cont	    ; continue

    steping

	step        ; step (into next function)
	next	    ; next statment
	finish      ; finish running function

    printing

	print x     ; print the value of x
	display  x  ; show the value of x every time stop


    Stack trace
	where (bt)  ;  Show active function calls
	up
	down

    stoping

	bye
	C-c         ; sends a signal to the program

  DDD  
    Graphical debugger based