C Reallocation of 2d array function - arrays

Question

My reallocation does not work (Segmentation fault for 11.element), I would like to enlarge the array twice, the length of the column is constant according to the first input.I would like to allocate in function.
In the function vztvorPole I allocate an array of 10 rows and x columns.
char vytvorPole(char ***grid, int nrows, int ncols)
{
*grid = malloc( sizeof(*grid)*nrows);
if (*grid == NULL)
{
printf("ERROR\n");
return 1;
}
for(int i=0;i<nrows;i++)
{
(*grid)[i]=(char *) malloc (ncols*sizeof(*grid));
if((*grid)[i] == NULL)
{
printf("ERROR\n");
return 1;
}
}
return 0;
}
char realokuj(char ***grid, int nrows, int ncols)
{
char **docasne;
docasne = (char**)realloc (*grid, nrows*sizeof(*grid));
for(int i=nrows/2;i<nrows;i++)
{
(docasne)[i]=(char *) malloc (ncols*sizeof(*grid));
}
*grid = docasne;
}
int main (void)
{
char **diagonaly;
int rDiagonaly = 10;
int cDiagonaly = -1;
char *str = NULL;
size_t capacity = 0;
int first = 1;
int nr = 0;
printf("Vypln:\n");
while ( getline (&str, &capacity, stdin) != -1)
{
if(str[0] == '\n')
break;
if (first)
{
cDiagonaly = strlen (str);
vytvorPole(&diagonaly, rDiagonaly, cDiagonaly);
first = 0;
}
if (nr==rDiagonaly)
{
rDiagonaly *= 2;
realokuj(&diagonaly, rDiagonaly, cDiagonaly);
}
strcpy(diagonaly[nr],str);
nr++;
}
}

Answer 1

Your code is a bit more complicated than needed.
If you initialize diagnonaly (e.g. char **diagnoaly = NULL;), you can eliminate vytvorPole and just use realokuj. That is, if you also do: int rDiagonaly = 0;
That's because realloc will handle a NULL pointer just fine (i.e. it's equivalent to malloc in that case).
And, you don't need to do:
(docasne)[i]=(char *) malloc (ncols*sizeof(*grid));
in the subfunction.
It's better to use strdup in main. If we do that, we don't need cDiagonaly at all. And, we can have strings of varying lengths.
Passing a triple star pointer (e.g. char ***grid) has issues. You could just pass it as char ** and use return to update the value since you don't really use the existing return.
But, this can really be done from main without a separate function.
While it may be possible to fix your existing code, it really should be simplified:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(void)
{
#if 0
char **diagonaly;
int rDiagonaly = 10;
#else
char **grid = NULL;
int rmax = 0;
#endif
#if 0
int cDiagonaly = -1;
#endif
char *str = NULL;
size_t capacity = 0;
#if 0
int first = 1;
#endif
int nr = 0;
printf("Vypln:\n");
while (getline(&str,&capacity,stdin) != -1) {
if (str[0] == '\n')
break;
if (nr == rmax) {
rmax += 100;
grid = realloc(grid,sizeof(*grid) * (rmax + 1));
if (grid == NULL) {
perror("realloc");
exit(1);
}
// this is similar to what you did, but it's not necessary because
// of "grid[nr] = NULL;" below
#if 0
for (int idx = nr; idx < rmax; ++idx)
grid[idx] = NULL;
#endif
}
grid[nr++] = strdup(str);
grid[nr] = NULL;
}
return 0;
}
Here's the cleaned up version [without the cpp conditionals]:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(void)
{
char **grid = NULL;
int rmax = 0;
char *str = NULL;
size_t capacity = 0;
int nr = 0;
printf("Vypln:\n");
while (getline(&str,&capacity,stdin) != -1) {
if (str[0] == '\n')
break;
if (nr == rmax) {
rmax += 100;
grid = realloc(grid,sizeof(*grid) * (rmax + 1));
if (grid == NULL) {
perror("realloc");
exit(1);
}
}
grid[nr++] = strdup(str);
grid[nr] = NULL;
}
return 0;
}

Answer 2

Is something like that okay?
while ( getline (&str, &capacity, stdin) != -1)
{
if(str[0] == '\n')
break;
if (first)
{
cDiagonaly = strlen (str);
diagonaly = (char**) malloc(sizeof(*diagonaly)*rDiagonaly);
if (nr==rDiagonaly)
{
rDiagonaly *= 2;
docasne = (char**)realloc (diagonaly, rDiagonaly*sizeof(*diagonaly));
diagonaly=docasne;
}
diagonaly[nr]=(char*) malloc (cDiagonaly*sizeof(**diagonaly));
strcpy(diagonaly[nr],str);
nr++;
}