Structure malloc problem

instinct46 · 09-07-2011, 12:30 PM

I'm having a problem where my opengl program is crashing, the reason why opengl wasn't in the title is because the error isn't a opengl related problem.

Their is something wrong with my code when I'm trying to malloc memory for a two-dimensional pointer. The problem is in the lines after the for loops, their are 2.

Code:

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

#include "objectheader.h"


void defobj(struct objContain *tmpstruc){
int loopa = 0;

    tmpstruc->faces = 1;
    tmpstruc->type = 1;

    tmpstruc->xcods == (int**) malloc(sizeof(int) * (tmpstruc->faces));
    for(loopa = 0; loopa < tmpstruc->faces; loopa++)
        tmpstruc->xcods[loopa] = (int*) malloc(sizeof(int) * (tmpstruc->type + 3));

    tmpstruc->ycods == malloc(sizeof(int) * (tmpstruc->faces));
    for(loopa = 0; loopa < tmpstruc->faces; loopa++)
        tmpstruc->ycods[loopa] = (int*) malloc(sizeof(int) * (tmpstruc->type + 3));
}

Code:

#ifndef _OBJECT_HEADER_H_
#define _OBJECT_HEADER_H_

#include <stdio.h>

struct objContain{
    unsigned int faces;
    int type; //TRIANGLE = 0 OR SQUARE = 1
    int **xcods;
    int **ycods;
};

#endif

paulsm4 · 09-07-2011, 12:35 PM

This looks wrong:

Code:

   tmpstruc->xcods == (int**) malloc(sizeof(int) * (tmpstruc->faces));
    for(loopa = 0; loopa < tmpstruc->faces; loopa++)
        tmpstruc->xcods[loopa] = (int*) malloc(sizeof(int) * (tmpstruc->type + 3));

Suggested alternative:

Code:

   tmpstruc->xcods = (int**) malloc(sizeof(int *) * (tmpstruc->faces));
    for(loopa = 0; loopa < tmpstruc->faces; loopa++)
        tmpstruc->xcods[loopa] = (int*) malloc(sizeof(int) * (tmpstruc->type + 3));

instinct46 · 09-07-2011, 12:39 PM

Haha cheers... might put the laptop down for a while and rest my eyes.

ambrop7 · 09-07-2011, 12:51 PM

This is still wrong. 'xcods' is declared as a pointer to an int pointer (int **). If you use it as a handle to a malloc'd array, the array must contain int pointers, not ints (which you're allocating).

Code:

tmpstruc->xcods = (int**) malloc(sizeof(int *) * (tmpstruc->faces));
for(loopa = 0; loopa < tmpstruc->faces; loopa++)
    tmpstruc->xcods[loopa] = (int*) malloc(sizeof(int) * (tmpstruc->type + 3));

The same can be achieved more nicely, avoiding such bugs:

Code:

tmpstruc->xcods = (int**) malloc(sizeof(tmpstruc->xcods[0]) * (tmpstruc->faces));
for(loopa = 0; loopa < tmpstruc->faces; loopa++)
    tmpstruc->xcods[loopa] = (int*) malloc(sizeof(tmpstruc->xcods[loopa][0]) * (tmpstruc->type + 3));

Also, in C, you don't need to cast the return value of malloc(), and any void pointer.

instinct46 · 09-07-2011, 01:30 PM

I only cast to keep the compiler from popping up with lots of warnings :P

I didn't know you could do sizeof(tmpstruc->xcods[0])

Cheers

ambrop7 · 09-07-2011, 01:40 PM

Quote:

Originally Posted by instinct46

I only cast to keep the compiler from popping up with lots of warnings :P

That's probably because you're compiling it as C++. If you don't use C++, you should compile it as C (with gcc not g++).