I just started to study C, and when doing one example about passing pointer to pointer as a function's parameter, I found a problem.
This is my sample code :
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int* allocateIntArray(int* ptr, int size){
if (ptr != NULL){
for (int i = 0; i < size; i++){
ptr[i] = i;
}
}
return ptr;
}
void increasePointer(int** ptr){
if (ptr != NULL){
*ptr += 1; /* <----------------------------- This is line 16 */
}
}
int main()
{
int* p1 = (int*)malloc(sizeof(int)* 10);
allocateIntArray(p1, 10);
for (int i = 0; i < 10; i++){
printf("%d\n", p1[i]);
}
increasePointer(&p1);
printf("%d\n", *p1);
p1--;
free(p1);
fgets(string, sizeof(string), stdin);
return 0;
}
The problem occurs in line 16, when I modify *ptr+=1
to *ptr++
. The expected result should be the whole array and number 1 but when I use *ptr++
the result is 0.
Is there any diffirence between +=1
and ++
? I thought that both of them are the same.
The difference is due to operator precedence.
The post-increment operator ++
has higher precedence than the dereference operator *
. So *ptr++
is equivalent to *(ptr++)
. In other words, the post increment modifies the pointer, not what it points to.
The assignment operator +=
has lower precedence than the dereference operator *
, so *ptr+=1
is equivalent to (*ptr)+=1
. In other words, the assignment operator modifies the value that the pointer points to, and does not change the pointer itself.