My C programming is rusty and i'm having some issues

Question

I am trying to declare a data structure in c and set some variables but I'm having a bit of trouble.

struct point {
   float *x;
   float *y;
   float *z;
};

this struct is 24 bytes long so that's fine by me.

const unsigned int sz = 1<<24;
struct point _points[sz];

for(int i = 0; i < sz; ++i)
{
    _points[i].x = get_rand_float(); 
    _points[i].y = get_rand_float();
    _points[i].z = get_rand_float();
}

// get_rand_float() returns a pointer to float;

The problem that I am having is that the application will crash.

I playing with the code a bit it seems that maybe 1<<24 is too large? Bringing it down to 1<<14 the program runs just fine.

That brings me to another question, why would 1<<24 or about 16 million ints cause my program to crash? It's a fairly trivial program just int main boilerplate and this struct?

Answer 1

In your code, sz is int variable, not an array. So, techinicaly you cannot use the array subscript operator on sz. That code should not compile.

Maybe, you wanted to write something like

 _points[i].x = get_rand_float();

But then again, it depends on get_rand_float() return type. It has to return a float * (which is not very likely seeing the function name).

In case, if get_rand_float() returns afloat value, and you want to store the returned value, then you don't need to use pointers as your structure member variable. You can simply use float x; and so on.

Answer 2

You don't want a structure of pointers to floats:

struct point {
   float *x;
   float *y;
   float *z;
};

You want a structure of floats:

struct point {
   float x;
   float y;
   float z;
};

Answer 3

EDIT: Based on your edit, your crash occurs when you use a massive struct size (1<<24), or 16,777,216 items in the struct. Borrowing an answer from here:

Size limitation of C structure

It would appear that you may be violating C standard by having over 65535 bytes in an object. Since 1<<14 works, which is only 16384, that might be why. To verify, try using anything above 1<<16 - those should all crash because it would be over 65535.

On a side note, it would be helpful if you post the actual error message you get so we have a better idea of what is going on. :)

---Pre-Author Edit Answer---

Assuming get_rand_float() returns what it's supposed to, the problem is that sz is an int, not a struct. It should look like:

int sz = 24;
struct point _points[sz];

for(int i = 0; i < sz; ++i)
{
    _points[i].x = get_rand_float(); 
    _points[i].y = get_rand_float();
    _points[i].z = get_rand_float();
}

Answer 4

Keeping apart the big size of the array,your program is mainly crashing because there no memory allocated to the pointer variables x,y and z. You have to allocate memory to the variables before assigning any values.

for(int i = 0; i < sz; ++i)
{
    sz[i].x = get_rand_float(); <--- getting crash here!
    sz[i].y = get_rand_float();
    sz[i].z = get_rand_float();
}

for(i=0;i<sz;i++)
    {
        _points[i].x =(float *) malloc(sizeof(float));
        _points[i].y = (float *) malloc(sizeof(float));
        _points[i].z = (float *) malloc(sizeof(float));
       *( _points[i].x) = get_rand_float();
       *( _points[i].y) = get_rand_float();
       *( _points[i].z) = get_rand_float();
    }
    for(i=0;i<sz;i++)
    {
        printf("%f %f %f ",*( _points[i].x), *(_points[i].y), *(_points[i].z));
        printf("\n");
    }

You can make your program simple by taking float as members of the structure instead of float pointers.

struct point {
   float x;
   float y;
   float z;
};
int main()
{
    int i;
    for(i=0;i<sz;i++)
    {
        _points[i].x = get_rand_float();
        _points[i].y = get_rand_float();
        _points[i].z = get_rand_float();
    }
    for(i=0;i<sz;i++)
    {
        printf("%f %f %f ", _points[i].x, _points[i].y, _points[i].z);
        printf("\n");
    }

Answer 5

Others have pointed out your two main problems (size too large, and you should be using float not float* in your struct). But there is another potential problem, too: you should not get into the habit of beginning an identifier name with an underscore, because, from Section 7.1.3 of the 1999 C standard:

• All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.

• All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

• Each macro name in any of the following subclauses (including the future library directions) is reserved for use as specified if any of its associated headers is included; unless explicitly stated otherwise (see 7.1.4).

• All identifiers with external linkage in any of the following subclauses (including the future library directions) are always reserved for use as identifiers with external linkage.154

• Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as a macro name and as an identifier with file scope in the same name space if any of its associated headers is included.

Answer 6

One possible problem is that your array of points is too large for the stack. See here. You could fix that by a dynamic memory allocation, something like:

struct point *_points = malloc(sz*sizeof(struct point));

And of course, don't forget to free the memory when finished.