Question

Hi I am trying to use C to implement a simple struct:
2 boxes, each contains different number of particles; the exact number of particles are passed in main().

I wrote the following code:

typedef struct Particle{
    float x;
    float y;
    float vx;
    float vy;
}Particle;

typedef struct Box{
    Particle p[];
}Box;

void make_box(Box *box, int number_of_particles);

int main(){
    Box b1, b2;
    make_box(&b1, 5);  //create a box containing 5 particles
    make_box(&b2, 10); //create a box containing 10 particles
}

I’ve tried to implement make_box with the following code

void make_box(struct Box *box, int no_of_particles){
    Particle po[no_of_particles];
    po[0].x = 1;
    po[1].x = 2;
    //so on and so forth...
    box->p = po;
}

It’s always giving me “invalid use of flexible array member”. Greatly appreciated if someone can shed some light on this.

Answer 1

void make_box(struct Box *box, int no_of_particles){
    Particle po[no_of_particles];
    //...
    box->p = po;
}

po is a local variable, and the storage for it is allocated automatically on the stack; returning its address is a bad idea. You should instead allocate memory from the heap, and remember freeing that memory when you’re done with the boxes:

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

typedef struct Particle_ {
    float x;
    float y;
    float vx;
    float vy;
} Particle;

typedef struct Box_ {
    Particle *p;
} Box;

void make_box(Box *box, int no_of_particles);

void make_box(Box *box, int no_of_particles){
    Particle *po = (Particle *) malloc ( no_of_particles*sizeof(Particle) );
    po[0].x = 1;
    po[1].y = 2;
    //so on and so forth...
    box->p = po;
}

void destroy_box(Box *box){
    free(box->p);
}


int main(){
    Box b1, b2;
    make_box(&b1, 5);  //create a box containing 5 particles
    make_box(&b2, 10); //create a box containing 10 particles

    // do the job...
    printf("box b1, point 0, x: %5.2f\n", b1.p[0].x);
    printf("box b2, point 1, y: %5.2f\n", b2.p[1].y);

    destroy_box(&b1);
    destroy_box(&b2);

    return 0;
}