** - Pointer to a pointer

[abraham2119]

Hello,

I am new to using Pointer to pointers rather than 2 dimensional arrays, but it is about time i needed dynamic memory.

Anyways, I typedef-ed a struct as follows:

typedef struct {
    float **vertex;
    float **face;
} MODEL;

And I have a function that is called from main() which takes a 'MODEL' pointer as parameters. I need to allocate memory for it to be similar to, if using arrays, array[3][valuesRecieve], and here is what I did:

 
    /* I want vertex to be [3][vertices]*/
   *(m->vertex) = malloc(sizeof(float)*3);
    m->vertex = malloc(sizeof(float)*vertices);

    /* I want face to be [2][faces]*/
    *(m->face) = malloc(sizeof(float)*2);
    m->face = malloc(sizeof(float)*faces);

But the program encounters error and exits. Any idea why?

PS: 'm' is the pointer to a MODEL in the parameters of the function.

Thank you,
abraham2119

[laserlight]

Since you have a fixed bound in both cases, it would be simpler to write:

typedef struct {
    float *vertex[3];
    float *face[2];
} Model;

Note that I renamed MODEL to Model since fully capitalised names are reserved for macros by convention.

Now, to dynamically allocate each of the inner arrays you might write something along the lines of:

m->vertex[0] = malloc(sizeof(*m->vertex[0]) * num_vertices);

The problem that you faced is partially due to allocating the wrong amount of space due to taking the sizeof the wrong thing, and partially due to doing things in the wrong order.

[abraham2119]

Since you have a fixed bound in both cases, it would be simpler to write:

typedef struct {
    float *vertex[3];
    float *face[2];
} Model;

Note that I renamed MODEL to Model since fully capitalised names are reserved for macros by convention.

Now, to dynamically allocate each of the inner arrays you might write something along the lines of:

m->vertex[0] = malloc(sizeof(*m->vertex[0]) * num_vertices);

The problem that you faced is partially due to allocating the wrong amount of space due to taking the sizeof the wrong thing, and partially due to doing things in the wrong order.

Thank you very much, but I think I did not express myself correctly.

I want to have an array(which size is dynamic) of float[3] for vertex and an array(which size is dynamic) of float[2] for face.

I think what you are giving me is the opposite, is it not?

[laserlight]

I want to have an array(which size is dynamic) of float[3] for vertex and an array(which size is dynamic) of float[2] for face.

I think what you are giving me is the opposite, is it not?

Yes, but you can change it to:

typedef struct {
    float (*vertex)[3];
    float (*face)[2];
} Model;

Likewise, the code for dynamic allocation would change to:

m->vertex = malloc(sizeof(*m->vertex) * num_vertices);

[abraham2119]

Yes, but you can change it to:

typedef struct {
    float (*vertex)[3];
    float (*face)[2];
} Model;

Likewise, the code for dynamic allocation would change to:

m->vertex = malloc(sizeof(*m->vertex) * num_vertices);

Thank you very much, but why is it that my code did not work?

And can you explain the logic of how (*vertex)[3] is the opposite of *vertex[3]?

Thank you!

[laserlight]

Thank you very much, but why is it that my code did not work?

• You dereference m->vertex before initialising it by writing *(m->vertex)
• You did not allocate memory for all the inner arrays
• You used sizeof(float) when sizeof(float*) was appropriate (though they might be equal, in which case it would merely be semantically incorrect)
• The same goes for m->face.

And can you explain the logic of how (*vertex)[3] is the opposite of *vertex[3]?

The syntax just means: "vertex is a pointer to an array of 3 floats" as opposed to "vertex is an array of 3 pointers to float".