populating array without calloc and malloc in c?

Question

I have code that looks like

int main(int argc, char *argv[]){
char ***matrix;
matrix = calloc(2, sizeof(char **));
for (int i=0; i<2; i++){
    matrix[i] = calloc(ROWS+2, sizeof(char*));
    for (int j=0; j<COLS; j++){
        matrix[i][j] = malloc(COLS+2);
        memset(matrix[i][j], (int)DEFAULT, COLS+2);
    }
}

Is there a way to do a similar kind of thing without the use of malloc and calloc? For example, in the case of 1d array I know you can do something like this

unsigned char malloc_data[MAX_SIZE];
size_t malloc_used;  /* automatically initialized to zero */

void *stack_malloc(size_t size) {
void *p = &malloc_data[malloc_used];
if(size + malloc_used > MAX_SIZE) return 0;  /* out of memory */
malloc_used += size;
return p;
}

OR something like this

static char *allocp = allocbuf[0];
if (allocbuf + ALLOCSIZE - allocp >= n) { /* it fits */ 

and

if (p >= allocbuf && p < allocbuf + ALLOCSIZE)

I want to try to use stack variables and temps instead of dynamic memory allocated ways. However, when I try to think about how to apply these types of ways to my three dimensional sense, my head starts spinning. Please help clarify things to me

Answer 1

Assuming you want to keep the triple pointer usage, the storage can be allocated on the stack and initialized like this:

char ***matrix;
char **mats_[2];
char *mat_rows_[2][ROWS+2];
char mat_cols_[2][ROWS+2][COLS+2];
matrix = &mats_[0];
for (int i = 0; i < 2; i++) {
    matrix[i] = &mat_rows_[i][0];
    for (int j = 0; j < ROWS+2; j++) {
        matrix[i][j] = &mat_cols_[i][j][0];
        memset(matrix[i][j], (int)DEFAULT, COLS+2);
    }
}

You could also define matrix as an array[2] of char ** instead of a single char *** , which would allow you to eliminate the mats_ variable:

char **matrix[2];
char *mat_rows_[2][ROWS+2];
char mat_cols_[2][ROWS+2][COLS+2];
for (int i = 0; i < 2; i++) {
    ...

---

In the comments below, OP asked whether the memset was necessary. All the elements of mat_cols_[2][ROWS+2][COLS+2] need to be set to the same value DEFAULT . If DEFAULT can be replaced with 0 , this is easy to do by defining mat_cols_ with an initializer with pretty much everything defaulting to 0:

char mat_cols_[2][ROWS+2][COLS+2] = { 0 };

If DEFAULT cannot be replaced with 0, the only way to do it would be for the initializer to supply values for all the elements explicitly, which has a maintainability problem since the dimensions are defined in terms of the macros ROWS and COLS . The GNU C Compiler (GCC) has an extension to the C language designated initializers that allows a range of array elements to be initialized to the same value. That could be used as follows:

char mat_cols_[2][ROWS+2][COLS+2] =
    {[0 ... 1] =
        {[0 ... ROWS+2-1] =
            {[0 ... COLS+2-1] = DEFAULT}
        }
    };