Question

I’m attempting to write a colloidal simulation program in C, and I came across a curious issue that I do not know how to resolve – I’ve reduced the code to the minimum where it shows the same type of error. (I’m sorry ahead of time for the length of the code…)

/** main.c **/
#include <stdio.h>

/** typedefs **/
typedef struct { double x, y, z; } point;
typedef struct {
    double radius;
    point pos, potential, velocity;
} particle;
typedef struct {
    int num;                // size of particle array
    particle *particles;
} trough;
typedef struct { trough trough; } experiment;

void printPoint(point a) { printf("%f, %f, %f\n", a.x, a.y, a.z); }

/** initializations **/
point initPoint(double x, double y, double z) {
    point new_point = { .x = x, .y = y, .z = z };
    return(new_point);
}
particle initParticle(double radius, point pos, point potential, point velocity) {
    particle new_particle = { .radius = radius, .pos = pos, .potential = potential, .velocity = velocity };
    return(new_particle);
}
trough initTrough(particle particles[]) {
    int num = sizeof(particles) / sizeof(int) + 1;
    trough new_trough = { .num = num, .particles = particles };
    return(new_trough);
}
experiment initExperiment(trough in_trough) {
    experiment new_experiment = { .trough = in_trough };
    return(new_experiment);
}

/** probs **/
experiment newExperiment() {
    particle new_part1 = initParticle(1, initPoint(0, 4, 0), initPoint(0, 0, 0), initPoint(0, 0, 0));
    particle new_part2 = initParticle(1, initPoint(4, 4, 0), initPoint(0, 0, 0), initPoint(0, 0, 0));

    printPoint(new_part1.pos);
    printPoint(new_part2.pos);

    particle particles[2] = { new_part1, new_part2 };
    trough new_trough = initTrough(particles);

    printPoint(new_trough.particles[0].pos);
    printPoint(new_trough.particles[1].pos);

    return(initExperiment(new_trough));
}
int main(int argc, char *argv[]) {
    experiment experiment = newExperiment();

    printPoint(experiment.trough.particles[0].pos);
    printPoint(experiment.trough.particles[1].pos);

    return(0);
}

I compile the code: gcc -lm -std=c99 -Wall main.c and run it: ./a.out, and get the following output:

$ ./a.out
0.000000, 4.000000, 0.000000
4.000000, 4.000000, 0.000000
0.000000, 4.000000, 0.000000
4.000000, 4.000000, 0.000000
0.000000, 4.000000, 0.000000
0.000000, 4.000000, 0.000000

The problem is the last line of output, where I was trying to find the x, y, z values of a point in the array of particles – all I did in the code was to print the values at various points, and so the values shouldn’t be changing – but they are, and I’m quite confused as to why this is. Would this have to do with referencing the array by index? Any help would be greatly appreciated.

Answer 1

Type trough does not contain an array of particles. It contains a pointer which can point at an array of particles. You initialized it to point at the function-local array declared in newExperiment().

But when function newExperiment ends, the array’s lifetime ends, and trying to use that dangling pointer again from main causes Undefined Behavior.

If there’s a known compile-time maximum to the number of particles, you can change the pointer member to an array member as @Patrick B showed. If not, you’ll need to use malloc and free to make sure the pointer member points at memory which will remain valid as long as you need it to.