Discussion
Assume integer is 2 bytes wide. What will be the output of the following code?
#include <stdio.h>
#include <stdlib.h>
#define MAXROW 3
#define MAXCOL 4
int main()
{
int (*p)[MAXCOL];
p = (int (*) [MAXCOL])malloc(MAXROW *sizeof(*p));
printf("%d, %d\n", sizeof(p), sizeof(*p));
return 0;
}
Answer: A
Correct answer: 2, 8
Two different quantities are being measured. sizeof(p) is the size of the pointer itself, which on the 16-bit platform assumed here is 2 bytes.
sizeof(*p) is the size of what it points to - an array of MAXCOL integers:
\[ 4 \times 2 = 8\ \mathrm{bytes} \]
The pair illustrates why sizeof on a pointer almost never gives what a beginner expects: it reports the size of the address, and the pointed-to type must be dereferenced to be measured.
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