XPM image memory allocation

Question

I am struggling with allocating memory for an XPM image.
I've created my own library with the following functions:

XPM* initXPM (

    unsigned int width,
    unsigned int height,
    unsigned int cpp,
    unsigned int ncolors )
{
    XPM *image;
    image=(XPM*)malloc(sizeof(Color*) * (width * height) * cpp);
    (*image).colta = (Color*) malloc(ncolors * sizeof(Color));
    (*image).width = width;
    (*image).height = height;
    (*image).cpp = cpp;
    (*image).ncolors = ncolors;

    int i;
    for (i = 0; i < ncolors; i++)
    {
        (*image).colta[i].pchars = (char*) malloc((cpp + 1) * sizeof(char));
    }

    (*image).data = (unsigned int**)malloc(height * sizeof(unsigned int*));
    for (i = 0; i < height; i++)
    {
        (*image).data[i] = (unsigned int*) malloc(width * sizeof(unsigned int));
    }

    printf("init :height : %d , width :%d ncolors:%d , cpp : %d\n",(*image).height,(*image).width,(*image).ncolors,(*image).cpp);
    return image;
}

The XPM structure looks like this :

typedef struct
{
    unsigned int r,g,b; /* the red, green and blue components */
    char *pchars; /* pointer to the character(s) combination */
} Color;

/* the number of characters for one color is contained into the XPM structure */

typedef struct
{
    unsigned int width; /* width in pixels */
    unsigned int height;
    unsigned int cpp; /* number of characters per pixel */
    unsigned int ncolors; /* how many colors we'll be using */
    Color *colta; /* colors array */
    unsigned int **data; /* the area that contains the image, */
    /* width x height locations for */
    /* indices into colta */
} XPM;

I am calling the init function in this way:

XPM *image;

    image = initXPM(200,200,1,2);

I have succesfully initialized the XPM file with the values :(50,50,2,50).If i try to initialize the file with 200 instead of 50 it crashes.
I want to create a 200x200 XPM file but it breaks. Using electricfence I found out that it crashes when i set the width of the image.
How should I allocate the memory in order to be able to use it like this?
I tried to tinker a bit with the structure and it works if I declare the structure statically in my main method and send the address of the XPM image to the init function, but I want to be able to use it like a library.

`(*image).colta` – ***nooooo!*** Just do `image->colta`. And [don't cast the return value of `malloc()`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc/605858#605858). — The Paramagnetic Croissant, Mar 02 '15 at 19:09
I've tried with `image->` instead of `(*image).` but it's the same result — Bogdan Molinger, Mar 02 '15 at 19:10
The line `image=(XPM*)malloc(sizeof(Color*) * (width * height) * cpp)` is very suspicious, due to the fact that you're not using `sizeof(XPM)` anywhere in it. — barak manos, Mar 02 '15 at 19:10
@Bogdan yeah, it's the same thing, it's just `->` is more readable (that's why I didn't post this as an answer but as a comment.) — The Paramagnetic Croissant, Mar 02 '15 at 19:11
@barakmanos exactly. and that's why `T *p = malloc(sizeof *p);` should be preferred over `T *p = malloc(sizeof(T));`. — The Paramagnetic Croissant, Mar 02 '15 at 19:12
@TheParamagneticCroissant: this line contains neither as far as I can tell. — barak manos, Mar 02 '15 at 19:12
@barakmanos no, it doesn't; it's probably a type mismatch due to a previous use of `sizeof(Color*)` (wherby the type of the allocated object has been changed). — The Paramagnetic Croissant, Mar 02 '15 at 19:13

score 0 · Answer 1 · answered Aug 27 '15 at 13:31

    image=(XPM*)malloc(sizeof(Color*) * (width * height) * cpp);

is clearly wrong, but since this allocates way too much memory for all but the tiniest images, it cannot cause the crash you mention. Nonetheless

image = malloc(sizeof *image); would be a correct allocation
initXPM(200, 200, 2, 50); does not crash on my system
you should use a debugger (such as gdb) rather than efence to analyze the problem.

XPM image memory allocation

1 Answers1