Segmentation Fault when filling a dynamical 2D array in C

Question

I try to create a 2D-Array and try to fill it with integers from a file. But when I run my program I get a Segmentation Fault at this line according to Valgrind:

*(array_valid_char++) = read_char;

Here is my code:

#include <stdio.h>
#include <stdlib.h>

int* f(char* name_file_in, int key_length)
{
    FILE *file_in;
    int* array_valid_char = malloc(95 * sizeof(int));//array of validated characters
    int count_char = 0;//count number of characters in file_in
    int read_char;//character being read

    file_in = fopen(name_file_in,"rb");

    if(file_in)
    {
        while ((read_char = fgetc(file_in)) != EOF)//browse the entire file
        {
            if ((count_char % key_length) == 0)//test if character's position in file belongs to the modulo of the key
            {
                for (int i = 0; i < 95; i++)//browse the entire array tabLatin
                {
                    *(array_valid_char++) = read_char;//read character put into array of validated characters
                }
            }
            count_char++;
        }
    }

    if(file_in){fclose(file_in);}

    return array_valid_char;
}

int** C1(char* name_file_in, int key_length)
{
    int **array_valid_keys = malloc(key_length * sizeof(int*));//array of array filled with all valid characters for a character of the key
    for (int i = 0; i < key_length; i++)
    {
        array_valid_keys[i] = malloc(95 * sizeof(int));
    }
    for (int i = 0; i < key_length; i++)
    {
        array_valid_keys[i] = f(name_file_in,i + 1);//for each character'position of the key is assigned an array given by f
    }
    return array_valid_keys;
}


int main(int argc,char* argv[])
{
    int key_length = atoi(argv[2]);
    int** array_valid_key = C1(argv[1], key_length);
    for(int i = 0; i < key_length; i++)
    {
        for(int j = 0; j < 95; j++)
        {
            printf("[%d] ",array_valid_key[i][j]); //printing all valid characters for each character of the key
        }
        printf("\n");
    }
    return(0);
}

What is wrong with that code?

If I understand correctly, a Segmentation Fault is an exceeding of memory allocated by the program. So my problem should be in the size of my 2D-Array or in how I try to fill it.

However, I have allocated a number of key_length columns like this:

int **array_valid_keys = malloc(key_length * sizeof(int*));

And, I have allocated each column to be an array of 95 cases with:

for (int i = 0; i < key_length; i++)
{
    array_valid_keys[i] = malloc(95 * sizeof(int));
}

When I fill this 2D-Array with

for (int i = 0; i < 95; i++)//browse the entire array tabLatin
{
    *(array_valid_char++) = read_char;
}

It shouldn't get an error because my loop is in the [1, 95] intervall (which is the size of each columns of my 2D-Array).

The other explanation is that one of my pointers are dangling according to this question but how?

Answer 1

Look at the following of your code snippet:

while ((read_char = fgetc(file_in)) != EOF)//browse the entire file
{
    if ((count_char % key_length) == 0)//test if character's position in file belongs to the modulo of the key
    {
        for (int i = 0; i < 95; i++)//browse the entire array tabLatin
        {
            *(array_valid_char++) = read_char;//read character put into array of validated characters
        }
    }
    count_char++;
}

The outer loop ( while loop) is for reading a single char, until you reach the end. Now in the inner loop ( for loop) you count from 0 to 94 (the loop ends if i reaches 95). Within that loop you are incrementing the pointer of array_valid_char .

If the test condition ( ((count_char % key_length) == 0) ) is valid for the first time you are filling all 95 elements of array_valid_char with the value of read_char .

But if the test condition is valid for the second time, you're doing the same thing. But now array_valid_char is outside of it's range. Because it's pointing to the 96. element of the array (which doesn't exists).