finding cause of segfault when accessing data in a struct

Question

I am getting a segfault when accessing anything within the point struct below from within another function. The printf before Calculate_values(&s) works as expected (along with other lines to test accessing the point struct), but a segmentation fault is caused when within the function.

Could this be a problem with pointers? log1 line is never reached, ruling out the call to Rpv function.

Code:

typedef struct point
{
    double *coord;
    double value;
} point;


typedef struct state
{
    void (*function)(point*, int);
    point *points; // p0, p1, p2, ...
    int dimentions; // number of points is always dimentions + 1
} state;

void Rpv(point *x, int dimentions)
{
    x->value = 0;

    for (int d = 0; d < dimentions - 1; d++)
        x->value += 100 * pow(x->coord[d + 1] - x->coord[d] * x->coord[d], 2) +
                          pow(1 - x->coord[d], 2);
}

void Calculate_values(state *s)
{
    for (int n = 0; n <= s->dimentions; n++)
    {
        printf("log (n: %d)\n", n);

        point p = s->points[0];

        printf("log1");

        s->function(p, s->dimentions);
        
        printf("log2\n");
    }
}


point minimisation(void (*function)(point*, int),
                   point initial_points[],
                   int dimentions)
{
    state s;
    s.points = initial_points;
    s.dimentions = dimentions;

    printf("[%lf, %lf] -> %lf\n", s.points[0].coord[0], s.points[0].coord[1], s.points[0].value);
    
    Calculate_values(&s);

    // ... more code

    return s.points[0];
}

int main()
{
    point initial_points[] = {
        {(double[]){3, 1}, 0}, // p1
        {(double[]){2, 0}, 0}, // p2
        {(double[]){0, 2}, 0}  // p3
    };

    point minimum = minimisation(Rpv, initial_points, 2);

    printf("minimum = %lf, @ (%lf, %lf)\n", minimum.value, minimum.coord[0], minimum.coord[1]);
    
    return 0;
}

Output seen:

[3.000000, 1.000000] -> 0.000000
log (n: 0)
Segmentation fault (core dumped)

Your debugging output doesn't flush properly, so it's unreliable. Please use a [*debugger*](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) to catch the crash and locate when and where in your code it happens. — Some programmer dude, Mar 23 '23 at 10:56
Your code does not compile. And where are your `#include`s? Please post your _actual_ code as [mcve]. — Jabberwocky, Mar 23 '23 at 10:58
in `minimisation()` you don't assign anything to `s.fiunction` but use the function pointer later — Ingo Leonhardt, Mar 23 '23 at 11:03

score 3 · Answer 1 · answered Mar 23 '23 at 11:05

Trouble-shooting seg faults:

Did I really initialize everything? state s;
Did I really allocate space for everything?
Do I calculate array indices with strange and poorly considered equations such as for (int d = 0; d < dimentions - 1; d++) ... x->coord[d + 1]
Does my code only contain proper, idiomatic for loops like for(int i=0; i<n; i++) or does it also contain "gimme out of bounds access" obfuscation loops like for (int n = 0; n <= ...

Once all such bad practices are weeded out, the bugs are probably gone too.

score 2 · Accepted Answer · answered Mar 23 '23 at 11:04

You never assign anything to s.function before calling Calculate_values. So getting a segfault here is expected. Technically it's undefined behaviour, but in this case the most likely outcome is a segfault

  ...
  state s;
  s.points = initial_points;
  s.dimentions = dimentions;
  s.function = Rpv;   // <<<<< add this line
  ...

I suggest you learn how to use your debugger. The time will quickly pay off. With a debugger you find this kind of bugs easily.

finding cause of segfault when accessing data in a struct

2 Answers2