c: Process finished with exit code 139 (interrupted by signal 11: SIGSEGV) - c

Question

I am making a program that can make an array from user input and save it inside a csv files. Then the user is able to perform matrix and vector operations with the stored arrays. When i started working with csv files(initially on clion) i started getting the error: "Process finished with exit code 139 (interrupted by signal 11: SIGSEGV)" After debugging i found the error "EXC_BAD_ACCESS (code=2, address=0x7ff7b202dff8)" . However the program still worked when ran in replit. Sadly though after i wrote lines 60 -65 in order to make a different csv file every time the function is ran i started getting the error: "signal: segmentation fault (core dumped) " on replit. What could be causing these errors? Here is my code on replit:
"https://replit.com/#iasonaszak/the-matrix#main.c"
Thank you in advance for your help!!
and here is my code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "time.h"
#include <math.h>
#include <stdbool.h>
int a;
char c[20];
int num;
bool b = true;
int main() {
while (b ==true){
void create_array();
void delete_array();
int user_answer;
time_t t;
srand((unsigned) time(&t));
printf(
" 1.create an array\n 2.load an array\n 3.Show available arrays\n 4.Delete array\n 5.Vector operations \n 6.Matrix operations"
) ;
printf("Please choose one of the following options!\n");
scanf("%d" , &user_answer);
if (user_answer == 1){
create_array( );
}
else if (user_answer == 4){
delete_array();
}
return 0;
}
}
void create_array(){
int rows;
int cols;
int i;
int j;
int matrix[i][j];
printf("how many columns would you like your array to have");
scanf("%d" ,&cols );
printf("how many rows would you like your array to have");
scanf("%d" ,&rows );
char filename[80];
FILE *fout = fopen(filename, "wt");
FILE *last = fopen("lastnum.csv" , "w");
fgets(c , 20 , last);
num = atoi(c);
snprintf(filename, sizeof(filename), "prefix.%d.csv", num);
for (i = 0 ; i < rows; i ++){
printf("\n");
fprintf(fout , "\n");
for (j = 0 ; j < cols; j ++){
scanf("%d", &matrix[i][j]);
char str[(int)((ceil(log10(matrix[i][j]))+1)*sizeof(char))];
sprintf(str , "%d " , matrix[i][j]);
fprintf(fout ,"%s" , str);
}
}
printf("Your array was saved successfuly inside a csv file");
num++;
}
void delete_array(){
remove("prefix.0.csv");
remove("prefix.1.csv");
remove("prefix.2.csv");
remove("prefix.3.csv");
remove("prefix.4.csv");
remove("prefix.5.csv");
remove("prefix.6.csv");
remove("prefix.7.csv");
remove("prefix.8.csv");
remove("prefix.9.csv");
printf("all arrays were deleted");
}
void preview_arrays(){
FILE *F0 = fopen("prefix.0.csv" , "r");
}

Answer 1

These three lines will cause a problem:
char str[(int)((ceil(log10(matrix[i][j]))+1)*sizeof(char))];
sprintf(str , "%d " , matrix[i][j]);
fprintf(fout ,"%s" , str);
The expression (int)((ceil(log10(matrix[i][j])) + 1 will produce 1 from a matrix value of 1, but char str[1]; won't be enough for the nul-terminated string.
And when the matrix value is <= 0 the logarithm is undefined. In my test with 0 it attempted to define char str[-2147483648];
You don't use str for anything else, so I suggest you remove those three lines and use this one simple line instead:
fprintf(fout,"%d ", matrix[i][j]);
Update
Another fault is in the file opening: wrong mode
FILE *last = fopen("lastnum.csv" , "w");
should be
FILE *last = fopen("lastnum.csv" , "r");
and always check that fopen() succeeds!
if(fout == NULL) {
/* handle error */
}
if(last == NULL) {
/* handle error */
}
and always check that fgets() succeeds too.
if(fgets(c , sizeof c , last)) == NULL) { // changed the 'magic' 20
/* handle error */
}