Lab #4 - Playing with pointers.

2/4/2004

This lab involves playing with a number of C programs to solve some problems involving pointers. There are 5 different problems, work on as many as you can. If you are a pointer guru, solve all the problems and make sure you get checked off before you leave...

Each program has instructions at the top. Each of these files are listed below (you can cut-and-paste), the files are also available in ~hollingd/public/lab4 on monte, the following command would copy the file p1.c from ~hollingd/public/lab4 to your current working directory:

cp ~hollingd/public/lab4/p1.c p1.c

IMPORTANT!: Ask for help if you don't understand something, some of the issues may not be obvious if you have not done lots with pointers.

---

p1.c

/* p1.c
 
   1. find out (add code to print out) the address of
   the variable x in foo1, and the variable y in foo2.

   2. add code to main that calls foo3 after the call to foo1
   and before the call to foo2. Rerun the program and see what happens.

*/

#include <stdio.h>

void foo1(int xval) {
  int x;
  x = xval;

  /* print the address and value of x here */
}

void foo2(int dmy) {
  int y;
  
  /* print the address and value of y here */
}

void foo3(char *s, int len) {
  int i,j,k;

  i=len+3; j=len+4;
}


int main() {
  foo1(3);
  /*  foo3("Hi Dave",5); */
  foo2(12);
}

---

p2.c

/* p2.c 
 
   1. test the function append by compiling and runing the program.
   Fix the problem (there are many ways to fix the problem, 
   ask for help if you can't figure out what the problem is).

   2. Rewite append so that it uses pointers instead of array
      syntax (instead of s1[i], use *s1, etc). Once again, 
	  ask for help if you don't know how to do this!

   3. Rewrite append so that it uses the library functions
      strcpy and strcat (at the command prompt type "man strcpy"
      or "man strcat" for details about these functions.
      You will need to add "#include <string.h>" to your program to
	  tell the compiler about these functions.

*/


#include <stdio.h>

/* function append appends two strings and returns the result
   (as a pointer to a new string)
   won't work unless the strings are no larger than
   10 chars each
*/

char *append(char *s1, char *s2) {
  char both[25];
  int i,j;

  if ((strlen(s1)>10) || (strlen(s2)>10)) {
	printf("Invalid string length (too big)\n");
	return(NULL);
  }

  i=0;
  /* copy the first string to both */
  while (s1[i]!=0) {
	both[i] = s1[i];
	i++;
  }

  /* now append the second string */
  j=0;
  while (s2[j]!=0) {
	both[i] = s2[j];
	i++;
	j++;
  }
 return(both);
}

int main() {

  char *s = append("Hello ", "World");
  if (s != NULL) 
	printf("Result is %s\n",s);
  return(0);
}

---

p3.c

/* p3.c

   1. compile and run the program and see what it prints out.

   2. Write some code that does pointer arithmetic with a 
      pointer to an int and determine how big an int is.

   3. Same idea - figure out how large the datatype double is
   by using pointer arithmetic and printing out a pointer before
   and after adding 3.


*/

#include <stdio.h>

int main( ) {

  char *s="abcdefghijklmnopqustuvwxyz";

  printf("s is %d   %08x\n",s,s);
  printf("The string at s is %s\n",s);

  /* Pointer arithmetic - see what s+3 is */
  s = s+3;
  printf("s is %d   %08x\n",s);

  /* and what is stored there (as a string) */
  printf("The string at s is %s\n",s);

}

---

p4.c

/* p4.c

   1. create a program that prints out the ASCII encoding of
   all the alphabetic characters and the space character ' '.
   You can print the ascii encoding of a char c with something like this:

   printf("%c  %2d  0x%02x\n",c,c,c);

   Note that you can do things like this:

   char c;
   for (c='a'; c<='z';c++) { ... }
   
   You need the output from this program to solve the question below.
   
   1.  For the source code in this file, 
   predict the value of i that is printed. Test your solution by
   compiling and runing the program, iterate until you
   can explain the result.
   Note that you need to determine the address of s and the
   address of i (or at least the relative difference)!

   2. fix the problem (make it so that it prints the
   correct value of i: "i is 0").
*/

#include <stdio.h>
#include <string.h>    /* needed for strcpy */

int main() {
  int i;
  char s[8];
  i=0;

  strcpy(s,"Pointers are lots of fun\n");
  printf("S is %s\n",s);
  printf("i is %d\n",i);
  return(0);
}

---

p5.c

/* p5.c

   swap_nums seems to work, but not swap_pointers. Fix it.

*/
#include <stdio.h>

void swap_nums(int *x, int *y) {
  int tmp;

  tmp = *x;
  *x = *y;
  *y = tmp;
}

void swap_pointers(char *x, char *y) {
  char *tmp;

  tmp = x;
  x = y;
  y = tmp;
}

int main() {
  int a,b;
  char *s1,*s2;

  a = 3; b=4;
  swap_nums(&a,&b);
  printf("a is %d\n",a);
  printf("b is %d\n",b);

  s1 = "I am s1 the great";
  s2 = "s2 rules";
  swap_pointers(s1,s2);
  printf("s1 is %s\n",s1);
  printf("s2 is %s\n",s2);

  return(0);
}

---

segvhunt.c

/* seghunt.c

   find and eliminate all code that generates segv

*/

#include <stdio.h>

int main() {
  char **s;
  char foo[] = "Hello World";
  
  *s = foo;
  printf("s is %s\n",s);

  s[0] = foo;
  printf("s[0] is %s\n",s[0]);

  return(0);
}