C - Segmentation fault for select values of fprintf - c

Question

I have a piece of code that calculates coordinates from a user defined size of grid, and prints them to a text file. However when I run the program I get a segmentation fault but only for values of 5 or 7 for the grid, all other values work as intended.
The code is as follows
// Writing OUTPUT 1 to file
// Creating & opening file
FILE *f1 = fopen("coords_output.txt", "w");
if(f1 == NULL)
{
printf("Error in opening file\n");
exit(0);
}
// Writing to file
for(i=0; i < arrayLength; i++)
{
for(j=0; j < arrayLength; j++)
{
fprintf(f1, "\n%d,%d %d", i, j, coordArray[i][j]);
}
}
// Close file
fclose(f1);here
If the whole section under //writing to file is removed the program works, and no matter what I place there the program will fail.
I don't have a great understanding of c, especially when it comes to areas like this, however I have spent a lot of time on trying to fix this problem and cannot seem to come to an answer.
EDIT:
coordArray is defined as
coordArray = malloc(arrayLength * sizeof(int *));
for(i=0; i <= arrayLength; i++)
{
coordArray[i] = malloc(arrayLength * sizeof(int));
}
arrayLength is taken from user input
validInput = validInput && sscanf(argv[1], "%d", &arrayLength);

Answer 1

This code doesn't work (modified from the code you provided):
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int arrayLength = 5;
int i,j;
int **coordArray;
coordArray = malloc(arrayLength * sizeof(int *));
for(i=0; i <= arrayLength; i++)
{
coordArray[i] = malloc(arrayLength * sizeof(int));
}
for(i=0; i < arrayLength; i++)
{
for(j=0; j < arrayLength; j++)
{
printf("\n%d,%d %d", i, j, coordArray[i][j]);
}
}
return 0;
}
It doesn't work because the first for loop is accessing 1 beyond the end of the array. Even though it may appear to work, the code is broken regardless of the value of arrayLength. The reason it may work for some values is due to how segmentation faults occur, there are lots of resources on the Internet about when and why they occur, here's a random one which seems to explain it well.
This code works (notice the change from <= to < in the first for loop):
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int arrayLength = 5;
int i,j;
int **coordArray;
coordArray = malloc(arrayLength * sizeof(int *));
for(i=0; i < arrayLength; i++)
{
coordArray[i] = malloc(arrayLength * sizeof(int));
}
for(i=0; i < arrayLength; i++)
{
for(j=0; j < arrayLength; j++)
{
printf("\n%d,%d %d", i, j, coordArray[i][j]);
}
}
return 0;
}
It can be difficult to find bugs like this without using a debugger or an analysis tool like Valgrind. I'd strongly recommend that you take the time to learn how to use tools like that so that you can find the source of these bugs in the future.