How can I store a variable length array in a struct in C

Question

I want to store a variable length array in a struct, and I use a pointer for this purpose. However, if I retrieve the stored array, I get the wrong values back. In the example below, I get the output "1 0", while you would expect the output "1 2".

#include <stdio.h>

typedef struct Flexibility {
    int *flex;
} Flexibility;

Flexibility calculateFlexibility()
{
    int a[2];
    a[0] = 1;
    a[1] = 2;

    Flexibility f;
    f.flex = a;
    return f;
}


void main()
{
    Flexibility f;
    f = calculateFlexibility();

    int i;
    for(i = 0; i < 2; i++)
    {
        fprintf(stdout, "%i ", *(f.flex + i));
    }

}

a gets out of scope when you return from calculateFlexibility, thus f->flex points to an array that doesn't exist anymore. — Matteo Italia
– Matteo Italia, Commented Oct 18, 2013 at 1:34

Iłya Bursov · Accepted Answer · 2013-10-21 19:37:37Z

4

you're creating temporary variable a in function calculateFlexibility, then you store pointer to f.flex variable, but after function is ended - a is gone from memory, so your f.flex pointer is now pointing to nowhere

if you want to have really variable length, you should do something like this:

Flexibility calculateFlexibility()
{
    Flexibility f;
    f.flex = (int*)malloc(....);
    return f;
}

and at the end of program:

free(f.flex);

for proper arguments of malloc I suggest you to read: http://en.cppreference.com/w/c/memory/malloc

edited Oct 21, 2013 at 19:37

user283145

answered Oct 18, 2013 at 1:37

Iłya Bursov

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