Dynamic allocation of an array of structs

Question

I've found useful answers on other people's questions countless times here on stackoverflow, but this is my first time asking a question of my own.

I have a C function that dynamically needs to allocate space for an array of structs and then fill the struct members of each array element with values pulled from a file. The member assignment works fine on the first pass of the loop, but I get a segmentation fault on the second pass.

I've written up this quick program illustrating the essentials of the problem I'm having:

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

typedef struct {
        int a;
        int b;
} myStruct;

void getData(int* count, myStruct** data) {
    *count = 5;
    *data = malloc(*count * sizeof(myStruct));

    int i;
    for (i = 0; i < *count; i++) {
        data[i]->a = i;
        data[i]->b = i * 2;
        printf("%d.a: %d\n", i, data[i]->a);
        printf("%d.b: %d\n", i, data[i]->b);
    }
}

int main() {
    int count;
    myStruct* data;
    getData(&count, &data);
    return 0;
}

The output I get from this is:

0.a: 0
0.b: 0
Segmentation fault

I'm not sure where my problem lies. It seems as though the malloc call is only allocating enough space for one struct when it should be allocating space for five.

Any help would be very much appreciated.

Mysticial · Accepted Answer · 2011-10-23 05:37:14Z

6

The error is here:

for (i = 0; i < *count; i++) {
    data[i]->a = i;
    data[i]->b = i * 2;
    printf("%d.a: %d\n", i, data[i]->a);
    printf("%d.b: %d\n", i, data[i]->b);
}

you should do this:

for (i = 0; i < *count; i++) {
    (*data)[i].a = i;
    (*data)[i].b = i * 2;
    printf("%d.a: %d\n", i, (*data)[i].a);
    printf("%d.b: %d\n", i, (*data)[i].b);
}

The reason is that you are indexing the wrong "dimension" of data.

answered Oct 23, 2011 at 5:37

Mysticial

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3 Comments

MahlerFive Over a year ago

You need to dereference data in the printfs too

David Thielke Over a year ago

Thanks for the incredibly quick response!

xappymah Over a year ago

I would say not a "wrong dimension" of data but a wrong array at all. As data[i] literally means "get i-th element of array of (myStruct*) (pointers to myStruct)" and the size of each element is a size of the pointer. And (*data)[i] is "i-th element of the (*data) array which is an array of flat myStruct structures"

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