C: how to manage a large structure?

Question

In C:

I am trying to use a structure containing a large array, and I have a stack overflow error while declaring it. I guess (correctly?) that I don't have enough memory in the stack, and therefore, I should use the heap (I don't want to change my stack memory size, as the code will be used by others). Could anyone show me a way to do it simply? Or should I use something else than a structure?

My code - definitions.h:

#define a_large_number 100000

struct std_calibrations{
    double E[a_large_number];
};

My code - main.c:

int main(int argc, char *argv[])
{
    /* ...
    */

    // Stack overflows here:
    struct std_calibrations calibration;

    /* ...
    */

    return (0);
} 

Thank you for your help!

Answer 1

A couple of options:

1. Use malloc(3) and free(3) to dynamically allocate your structure at runtime. This option is what you're talking about when you say you "should use the heap."

   struct std_calibrations *calibration = malloc(sizeof *calibration);
   

   and later,

   free(calibration);
   

2. Give the structure static storage duration. Either add a static keyword or make it global. This option may change some semantics about how you use the structure, but given your example code, it should be fine.

Answer 2

Change the member E to be a double* and malloc() memory for it:

struct std_calibrations calibration;
calibration->E = malloc(sizeof(*calibration->E) * a_large_number);

and remember to free(calibration->E); when no longer required. It would be possible to extend struct std_calibrations, if desired, to contain the number of elements in E so a user of struct std_calibrations could decide how many elements they require.