null pointer passed as argument 1

Question

I had written this function for argument checking but it is showing this type of error

vigenere.c:26:20: runtime error: null pointer passed as argument 1, which is declared to never be null
/usr/include/string.h:400:33: note: nonnull attribute specified here
Segmentation fault

my code

int check (char arg[])
{
    int j = 0;
    int k = strlen(arg);
    for (int i = 0; i < k; i++)
    {
        if(isdigit(arg[i]))
        {
            j = 1;
        }
    }
return j;
}

Answer 1

You forgot to include the relevant code.

You probably call your check function with a NULL pointer. This might happen for example if you use argv[1] before checking whether argc is greater than 1 (or here: equals 2), otherwise that array element wouldn't exist, and could have any value.