C Pointers
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CIS-Linux2:~/2012/c/ptrs>more pointer1.c
/* "&" is the "address of" operator, "*" is the "thing pointed to by" operator 
#include <stdio.h>   
main() {
   int i;
   int *pi;                  /* *pi is an int    */
   i = 10;
   pi = &i;                  /* pi points to i   */
   printf("%8.8x\n", i);     /* printed 0000000a */        
   printf("%8.8x\n", *pi);   /* printed 0000000a */        
   printf("%8.8x\n", pi);    /* printed 8a205dac */             
}
CIS-Linux2:~/2012/c/ptrs>gcc pointer1.c
CIS-Linux2:~/2012/c/ptrs>./a.out
0000000a
0000000a
8a205dac
CIS-Linux2:~/2012/c/ptrs>
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CIS-Linux2:~/2012/c/ptrs>more pointer2.c
#include <stdio.h>   
main() {
   int   i,   j,   k,   l;
   int *pi, *pj, *pk, *pl;                                /* *pi is an int    */
   i = 16, j=32, k=48, l=64;
   pi = &i, pj = &j, pk = &k, pl = &l;                    /* pi points to i   */
   printf("   i=%8.8x   j=%8.8x   k=%8.8x   l=%8.8x\n",    i,    j,    k,    l);
   printf("  pi=%8.8x  pj=%8.8x  pk=%8.8x  pl=%8.8x\n",   pi,   pj,   pk,   pl);
   printf(" *pi=%8.8x *pj=%8.8x *pk=%8.8x *pl=%8.8x\n",  *pi,  *pj,  *pk,  *pl);
}
CIS-Linux2:~/2012/c/ptrs>gcc pointer2.c
CIS-Linux2:~/2012/c/ptrs>./a.out
   i=00000010   j=00000020   k=00000030   l=00000040
  pi=e03bda9c  pj=e03bda98  pk=e03bda94  pl=e03bda90
 *pi=00000010 *pj=00000020 *pk=00000030 *pl=00000040
CIS-Linux2:~/2012/c/ptrs>
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CIS-Linux2:~/2012/c/ptrs>more pointer3.c
/* C always passes copies of arguments to functions */
#include <stdio.h>
void exch(int, int);    
main() {
   int   i,   j;
   i = 16, j=32;
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
   exch(i, j);
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
}
void exch(int i, int j) {
   int temp;
   temp = i;
   i = j;
   j = temp;
}
CIS-Linux2:~/2012/c/ptrs>gcc pointer3.c
CIS-Linux2:~/2012/c/ptrs>./a.out
   i=00000010   j=00000020
   i=00000010   j=00000020
======================================================================================
CIS-Linux2:~/2012/c/ptrs>more pointer4.c
/* Exchanging copies of the pointers isn't going to work either */
#include <stdio.h>
void exch(int *, int *);    
main() {
   int i, j;
   i = 16, j=32;
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
   exch(&i, &j);
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
}
void exch(int *pi, int *pj) {
   int *ptemp;
   ptemp = pi;
   pi = pj;
   pj = ptemp;
}
CIS-Linux2:~/2012/c/ptrs>gcc pointer4.c
CIS-Linux2:~/2012/c/ptrs>./a.out
   i=00000010   j=00000020
   i=00000010   j=00000020
CIS-Linux2:~/2012/c/ptrs>
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CIS-Linux2:~/2012/c/ptrs>more pointer5.c
/* This blows up, but why? */ 
#include <stdio.h>
void exch(int *, int *);    
main() {
   int   i,   j;
   i = 16, j=32;
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
   exch(&i, &j);
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
}
void exch(int *pi, int *pj) {
   int *ptemp;
   *ptemp = *pi;
   *pi = *pj;
   *pj = *ptemp;
}
CIS-Linux2:~/2012/c/ptrs>gcc pointer5.c
CIS-Linux2:~/2012/c/ptrs>./a.out
   i=00000010   j=00000020
   i=00000020   j=00000010
Segmentation fault
CIS-Linux2:~/2012/c/ptrs>
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CIS-Linux2:~/2012/c/ptrs>more pointer6.c
/* Finally, either of the following functions will work */ 
#include <stdio.h>
void exch(int *, int *);    
void exchv2(int *, int *);    
main() {
   int   i,   j;
   i = 16, j=32;
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
   exch(&i, &j);
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
}
void exch(int *pi, int *pj) {
   int temp;
   temp = *pi;
   *pi = *pj;
   *pj = temp;
}
void exchv2(int *pi, int *pj) {
// using unneeded pointer to store the int
   int temp, *ptemp;
   ptemp = &temp;
   *ptemp = *pi;
   *pi = *pj;
   *pj = *ptemp;
}
CIS-Linux2:~/2012/c/ptrs>gcc pointer6.c
CIS-Linux2:~/2012/c/ptrs>./a.out
   i=00000010   j=00000020
   i=00000020   j=00000020
CIS-Linux2:~/2012/c/ptrs>
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CIS-Linux2:~/2012/c/ptrs>more pointer7.c
/* Or, if you like, you can exchange pointers */ 
#include <stdio.h>
void exch(int **, int **);    
main() {
   int   i,    j;
   int *pi,   *pj;
   i  = 16,  j=32;
   pi = &i, pj=&j;
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
   printf("  pi=%8.8x  pj=%8.8x\n",   pi,   pj);
   printf(" *pi=%8.8x *pj=%8.8x\n",  *pi,  *pj);
   exch(&pi, &pj);
   printf("   i=%8.8x   j=%8.8x\n",    i,    j);
   printf("  pi=%8.8x  pj=%8.8x\n",   pi,   pj);
   printf(" *pi=%8.8x *pj=%8.8x\n",  *pi,  *pj);
}
void exch(int **ppi, int **ppj) {
   int *ptemp;
   ptemp = *ppi;
   *ppi = *ppj;
   *ppj = ptemp;
}
CIS-Linux2:~/2012/c/ptrs>gcc pointer7.c
CIS-Linux2:~/2012/c/ptrs>./a.out
   i=00000010   j=00000020
  pi=5311c98c  pj=5311c988
 *pi=00000010 *pj=00000020
   i=00000010   j=00000020
  pi=5311c988  pj=5311c98c
 *pi=00000020 *pj=00000010
CIS-Linux2:~/2012/c/ptrs>
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