Unable to access struct array members by pointer index

Question

I am trying to accessing data from a global array of structs. However the following application crashes.

#include <stdio.h>

typedef struct {
    char *fruit;
    int score;
} t_fruitdata;

static t_fruitdata fruittable[] = {
    {"Apple", 100},
    {"Banana", 240},
    {"Carrot", 40}
};

void main()
{
    int table_len = sizeof(fruittable) / sizeof(t_fruitdata);
    for (int i = 0; i < table_len; ++i)
    {
        t_fruitdata *fruitdata = fruittable + i*sizeof(t_fruitdata);
        printf("At index %i fruit %s has a score of %i\n",
            i,
            fruitdata->fruit,
            fruitdata->score);
    }
}

Outputs:

At index 0 fruit Apple has a score of 100
[program terminates here]

I imagine I've stumbled into some undefined behavior? But I've seen this technique recommended on stack overflow before. I'm using cl on windows to compile this with Visual Code 2017.

fruittable + i*sizeof(t_fruitdata) -> fruittable + i. Pointer arithmetics. — Eugene Sh.
– Eugene Sh., Commented Jul 26, 2018 at 19:50
More specifically, t_fruitdata *fruitdata = fruittable + i treats i as an index of t_fruitdata structs, so in the statement I've written, i is automatically multiplied by sizeof(t_fruitdata) when evaluating where fruitdata points. This in turn means that you don't need to do the multiplication yourself. — dgnuff
– dgnuff, Commented Jul 26, 2018 at 20:27

John Bollinger · Accepted Answer · 2018-07-26 20:27:45Z

1

In t_fruitdata *fruitdata = fruittable + i*sizeof(t_fruitdata); you are not correctly incrementing the pointer.

#include <stdio.h>

typedef struct {
    char *fruit;
    int score;
} t_fruitdata;

static t_fruitdata fruittable[] = {
    {"Apple", 100},
    {"Banana", 240},
    {"Carrot", 40}
};

void main()
{
    int table_len = sizeof(fruittable) / sizeof(t_fruitdata);
  t_fruitdata *fruitdata = fruittable;

    for (int i = 0; i < table_len; ++i)
    {
           printf("At index %i fruit %s has a score of %i\n",
            i,
            fruitdata->fruit,
            fruitdata->score);
fruitdata++;
    }
}

edited Jul 26, 2018 at 20:27

John Bollinger

190k11 gold badges102 silver badges206 bronze badges

answered Jul 26, 2018 at 20:05

kiran Biradar

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2 Comments

David C. Rankin Over a year ago

The proper declarations for main are int main (void) and int main (int argc, char **argv) (which you will see written with the equivalent char *argv[]). note: main is a function of type int and it returns a value. See: C11 Standard §5.1.2.2.1 Program startup p1 (draft n1570). See also: See What should main() return in C and C++?

Murillo Ferreira Over a year ago

I'd rather use fruitdata[i] or (fruitdata + i) instead of fruitdata++ to avoid any mistakes since you are changing where the pointer fruitdata is pointing.

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