Am I returning double char the correct way?

Question

I am currently practicing malloc and trying to create an array of strings in c. The following is my little program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int read_arguments(char*s[]);
char** copy_argv(char*s[]);
void free_char_ary(char*s[]);
int main(int argc, char* argv[])
{
    int count = read_arguments(argv);
    
    char **arr = copy_argv(argv);
    for(int i = 0; i < count; i++)
    {
        printf("%s\n", arr[i]);
    }

    free_char_ary(arr);
    exit(0);
}

int read_arguments(char*s[])
{
    int count = 0;
    
    while(*s) 
    {
        count++;
        s++;
    }
    
    return count;
}


char** copy_argv(char*s[])
{
    int result = read_arguments(s);
    printf("result = %d\n", result);
    char** ary = (char**) malloc(result * sizeof(char*));
    for(int i = 0; i < result; i++)
    {
        ary[i] = (char*) malloc(100 * sizeof(char));
        strcpy(ary[i], s[i]);        
    } 
    return ary;
}

void free_char_ary(char*s[])
{
    int count = read_arguments(s);
    printf("count = %d\n", count);
    for(int i = 0; i < count; i++)
    {
        free(s[i]);
    }
    free(s);
}

The result is weird. If i execute for like 4 arguments it is fine, but if i execute with 5 arguments then i get segmentation fault at the free_char_ary. I found that the int returned by read_arguments is different after i copy_argv to char**arr. Am I using the double char pointer the correct way? Why is the result different?

Answer 1

The function free_char_ary has undefined behavior because the dynamically allocated array does not contain an element with the value NULL. As a result the call of read_arguments within the function invokes the undefined behavior.

void free_char_ary(char*s[])
{
    int count = read_arguments(s);
    printf("count = %d\n", count);
    for(int i = 0; i < count; i++)
    {
        free(s[i]);
    }
    free(s);
}

You should append the dynamically allocated array with a null pointer the same way as the array argv is defined. Or you could pass to the function the actual number of elements in the dynamically allocated array,