Two-dimensional string arrangement using pointers in C - c

Question

I want to do a two-dimensional array of strings using pointers, since by doing that I can later release the dynamic memory and make a smaller or larger array using the same identifier (variable).
I used the code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
unsigned long rows = 0;
unsigned long cols = 0;
unsigned long chrs = 30;
char ***mtx = NULL;
int rszmtx(char ***, unsigned long, unsigned long);
int main(int argc, char *argv[]) {
rszmtx(mtx, 0, 0);
for (int r = 0; r < rows; r++) {
for (int c = 0; c < cols; c++) {
strcpy(mtx[r][c], "[]");
}
}
for (int r = 0; r < rows; r++) {
for (int c = 0; c < cols; c++) {
printf("%s ", mtx[r][c]);
}
printf("\n");
}
return 0;
}
int rszmtx(char ***matrix, unsigned long y, unsigned long x) {
if (y <= 0) {
printf("Enter the number of rows: ");
scanf("%lu", &rows);
} else {
y = rows;
}
if (x <= 0) {
printf("Enter the number of cols: ");
scanf("%lu", &cols);
} else {
x = cols;
}
free(matrix);
matrix = (char ***) malloc(rows * sizeof(char **));
if (matrix == NULL)
return 1;
for (int r = 0; r < rows; r++) {
matrix[r] = (char **) malloc(cols * sizeof(char *));
for (int c = 0; c < cols; c++) {
matrix[r][c] = (char *) malloc (chrs * sizeof(char));
}
}
return 0;
}
In the function "main" I want to call the function "rszmtx" and assign a size to "mtx" given by the user. Then I want all the strings to be "[]" and print them out. The problem is that I can neither assign nor print anything. When testing I noticed that if I put the mapping and/or printing inside the "rszmtx" function everything works fine. Why is this, how can I map and print "mtx" outside the "rszmtx" function?

Answer 1

You have to pass the pointer by reference. For example
int rszmtx(char ****mtx, unsigned long y, unsigned long x);
//…
rszmtx(&mtx, 0, 0);
Otherwise the function deals with a copy of the original pointer mtx and changing a copy in the function does not influence on the original; pointer.
And there is no sense to define the pointer globally.

Answer 2

If it is a string matrix I would not do it like your solution with ********* pointers, only create set of functions like this:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdio.h>
#define SFUNC 1
typedef struct
{
int malloced;
char *string;
}STRING_t;
typedef struct
{
size_t xsize;
size_t ysize;
STRING_t strings[];
}STR2D_t;
STR2D_t *init(size_t xsize, size_t ysize)
{
STR2D_t *str2D = calloc(sizeof(str2D) + xsize * ysize * sizeof(str2D -> strings[0]), 1);
str2D -> xsize = xsize;
str2D -> ysize = ysize;
return str2D;
}
char *setstr2D(STR2D_t *str2D, const char *str, size_t xpos, size_t ypos)
{
char *result = NULL;
#if SFUNC
if(str2D && str2D -> xsize > xpos && str2D -> ysize > ypos)
#endif
{
STRING_t *strStruct = &str2D -> strings[xpos + ypos * str2D -> ysize];
#if SFUNC
if(strStruct -> malloced)
{
free(strStruct -> string);
strStruct -> malloced = 0;
}
#endif
strStruct -> string = (char *)str;
result = (char *)str;
}
return result;
}
char *strcpy2D(STR2D_t *str2D, const char *str, size_t xpos, size_t ypos)
{
char *result = NULL;
#if SFUNC
if(str2D && str2D -> xsize > xpos && str2D -> ysize > ypos)
#endif
{
STRING_t *strStruct = &str2D -> strings[xpos + ypos * str2D -> ysize];
if(strStruct -> string) strcpy(strStruct -> string, str);
result = strStruct -> string;
}
return result;
}
char *strmalloc2D(STR2D_t *str2D, size_t size, size_t xpos, size_t ypos)
{
char *result = NULL;
#if SFUNC
if(str2D && str2D -> xsize > xpos && str2D -> ysize > ypos)
#endif
{
STRING_t *strStruct = &str2D -> strings[xpos + ypos * str2D -> ysize];
result = realloc(strStruct -> malloced ? strStruct -> string : NULL, size);
if(result)
{
strStruct -> string = result;
strStruct -> malloced = 1;
}
}
return result;
}
char *strdup2D(STR2D_t *str2D, const char *str, size_t xpos, size_t ypos)
{
char *result = strmalloc2D(str2D, strlen(str) + 1, xpos, ypos);
if(result) result = strcpy2D(str2D, str, xpos, ypos);
return result;
}