You are mixing up various coding styles here. It's not clear what exactly you want to achieve. Pick one according to your task.
Temporary buffer
You need a large temporary buffer that should be allocated on the heap and that does not need to be seen from outside. Just create a local variable:
void do_stuff(int w, int h)
{
coord **p;
int i;
p = malloc(h * sizeof(*p));
for (i = 0; i < h; i++) p[i] = malloc(w * sizeof(**p));;
// do stuff
for (i = 0; i < h; i++) free(p[i]);
free(p);
}
Allocate memory for further use
You want to allocate storage that your client code can use. Then provide two functions, one that allocates and one that frees the memory:
coord **create(int w, int h)
{
coord **p;
int i;
p = malloc(h * sizeof(*p));
for (i = 0; i < h; i++) p[i] = malloc(w * sizeof(**p));
return p;
}
void destroy(coord **p, int h)
{
int i;
for (i = 0; i < h; i++) free(p[i]);
free(p);
}
Your client code can then use the memory between these calls:
coord **p = create(500, 460);
// do stuff
drestroy(p, 500);
(Note that you have to pass the height to destroy
, which is a bit unfortunate. It might be cleaner to create a wrapper struct that hold information about width and height and the pointer.)
Allocate memory for a global variable
You have a single instance of a global pointer. Then your functions always operate on that pointer and you don't need any further information on it (except the dimensions):
coord **global = NULL;
void destroy_global(int h)
{
int i;
for (i = 0; i < h; i++) free(global[i]);
free(global);
global = NULL;
}
void create_global(int w, int h)
{
int i;
if (global != NULL) free_global();
global = alloc(h * sizeof(*global));
for (i = 0; i < h; i++) global[i] = malloc(w * sizeof(**global));
}
Note that you should include <stdlib.h>
for all memory functions and the NULL
macro.
Addendum According to your comment, you want to allocate memory for a bitmap. That's option 2 above.
I recommend to create an object structure. You can pass a pointerv to that structure as handle to a bunch of functions. You can create the object with a function that returns that handle.
The following sketches a rough design for a bitmap object.
typedef struct Pixel Pixel;
typedef struct Bitmap Bitmap;
struct Pixel {
uint8_t r, g, b;
};
struct Bitmap {
int height;
int width;
Pixel **pixel;
};
Bitmap *bitmap_new(int w, int h)
{
Bitmap *bmp = malloc(sizeof(*bmp));
int i;
bmp->height = h;
bmp->width = w;
bmp->pixel = malloc(h * sizeof(*bmp->pixel));
for (i = 0; i < h; i++) {
bmp->pixel[i] = malloc(w * sizeof(**bmp->pixel));
}
return p;
}
void bitmap_delete(Bitmap *bmp)
{
int i;
for (i = 0; i < h; i++) free(bmp->pixel[i]);
free(bmp->pixel);
free(bmp);
}
Bitmap *bitmap_read(const char *fn)
{
Bitmap *bmp;
FILE *f = fopen(fn, "rb");
// read and allocate
return bmp;
}
void bitmap_blank(Bitmap *bmp, int r, int g, int b)
{
for (i = 0; i < bitmap->height; i++) {
for (j = 0; j < bitmap->width; j++) {
bmp->pixel[i][j].r = r;
bmp->pixel[i][j].g = g;
bmp->pixel[i][j].b = b;
}
}
}
void bitmap_mirror_x(Bitmap *bmp)
{
// do stuff
}
int bitmap_write(Bitmap *bmp, const char *fn)
{
FILE *f = fopen(fn, "rb");
// write bitmap to file
return 0;
}
The design is similar to the interface to FILE *
: fopen
gives you a handle (or NULL
; error checking is omitted in the code above) and fread
, fprintf
, fseek
and family take a pointer to the file as argument. Finally call fclose
to close the file on disk and to free any ressources fopen
has claimed.