Iterating struct array by pointer arithmetic

Question

I'm trying to iterate over an array of structs by pointer arithmetic. However it gives me results I can't understand.

#include <stdio.h>
#include <stdlib.h>

typedef struct
{
    int hours;
    int minutes;
    int seconds;
} Time;

void time_print(Time t)
{
    printf("Time is: %d:%d:%d\n", t.hours, t.minutes, t.seconds);
}

int main(void)
{    
    Time *testTimePointers[2];

    testTimePointers[0] = malloc(sizeof(Time));
    testTimePointers[0]->hours = 11;
    testTimePointers[0]->minutes = 10;
    testTimePointers[0]->seconds = 9;

    testTimePointers[1] = malloc(sizeof(Time));
    testTimePointers[1]->hours = 7;
    testTimePointers[1]->minutes = 6;
    testTimePointers[1]->seconds = 5;

    time_print(*(testTimePointers[0]));
    time_print(*(testTimePointers[1]));

    printf("=============\n");


    Time *ttp_cur = NULL;
    ttp_cur = testTimePointers[0];

    time_print(*(ttp_cur));

    ttp_cur++;

    time_print(*(ttp_cur));


    free(testTimePointers[0]);
    free(testTimePointers[1]);

    return 0;
}

Results in

Time is: 11:10:9 
Time is: 7:6:5
=============
Time is: 11:10:9
Time is: 0:0:0

In the second block, the second line should be Time is: 7:6:5 instead of Time is: 0:0:0.

After ttp_cur++; pointer makes step equal to sizeof(Time) and points to nowhere. — tema
– tema, Commented Apr 13, 2015 at 12:00

Sergey Kalinichenko · Accepted Answer · 2015-04-13 11:59:40Z

7

The problem is that you are trying to increment a pointer ttp_cur that does not point to an array.

The value you are trying to increment, ttp_cur, came from testTimePointers[0], which in turn came from malloc of a single struct Time, not an array. That's why when you increment that pointer, the behavior is undefined.

It would be OK to increment a pointer pointing to testTimePointers array, however. If you did this

Time **ttp_cur_p = testTimePointers;
time_print(*(*ttp_cur_p));
ttp_cur_p++;
time_print(*(*ttp_cur_p));

you would get the results that you expect (demo).

answered Apr 13, 2015 at 11:59

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Vlad from Moscow · Accepted Answer · 2015-04-13 12:07:59Z

I think that in the second part of the program you mean the following

    //...

    Time **ttp_cur = testTimePointers;

    time_print(**ttp_cur );

    ttp_cur++;

    time_print(**ttp_cur);


    free(testTimePointers[0]);
    free(testTimePointers[1]);

    return 0;
}

If you have an array like

T a[2];

where T is some type specifier then the pointer to its first element is declared the following way

T *p = a;

and expression

++p;

will point to the second element of the array.

Now if to consider your array

Time *testTimePointers[2];

then type specifier T will be equal to Time *. So the declaration of the pointer to the first element of the array will look like

Time **ttp_cur = testTimePointers;

and expression

++ttp_cur;

will point to the second element of the array.

Collectives™ on Stack Overflow

Iterating struct array by pointer arithmetic

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related