pointer problem

**Cprogrammer** · 01-03-2004

Hi all,
I have a problem with this code. Can somebody explain to me what this line does p = (char (*)[SZ])in;
I know that "in" is beeing casted but its strange when i see it with pointers. What is exactly happening in memory, what is p pointing at?

And also what is the difference between
char (*p)[SZ]; and char *p[SZ];

thx.

Code:

#include <stdio.h>

#define SZ 3
char print(char c) {
   return c >= ' ' && c <= '~' ? c : '#';
}

void fooey(char *in, char *out, int size) {
   char (*p)[SZ]; /*p points to array[s] of SZ chars each */
   int r, c, i;

   p = (char (*)[SZ])in;

   for(c = i = 0; c < SZ; c++)
      for(r = 0; r < size/SZ; r++, i++) {
		 out[i] = p[r][c] && p[r][c] != ' ' ? (p[r][c] | ' ') + 1 : p[r][c];
		 printf("%d=%c,(%d,%d)=%c\n", i, print(out[i]), r, c, print(p[r][c]));
       }
}

int main( ) {
   char test[9] = "NFCGNX N",
	test2[sizeof test];

   fooey(test, test2, sizeof test);
   printf("%s\n", test2);
   return 0;
}

**Dave_Sinkula** · 01-03-2004

The cast is used because 'in' is a pointer to char. It may be indexed like an array, but only in one dimension. The cast allows 'p' to be indexed as a two-dimensional array. For example, you can't do the following.

Code:

void fooey(char *in, char *out, int size) {
   int r, c, i;
   for(c = i = 0; c < SZ; c++)
      for(r = 0; r < size/SZ; r++, i++) {
		 out[i] = in[r][c] && in[r][c] != ' ' ? (in[r][c] | ' ') + 1 : in[r][c];
		 printf("%d=%c,(%d,%d)=%c\n", i, print(out[i]), r, c, print(in[r][c]));
       }
}

Using 'p' allows the two-dimensional indexing. But I don't believe this is in strict conformance with the standard (reference).

>And also what is the difference between
>char (*p)[SZ]; and char *p[SZ];

Code:

char (*p)[SZ]; /* p is a pointer to an array of SZ chars */
char  *q [SZ]; /* q is an array of SZ pointers to char */

More info can be found here (near the bottom). It is a chapter from A TUTORIAL ON POINTERS AND ARRAYS IN C, which contains lots of good information. And even more information can be found here.

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