Brainfuck interpreter not running some codes

I'm kinda new to C programming and decided that making a brainfuck interpreter in C would be a good way to learn the language. I could write and tested with these bf codes:

this should print a hello world

++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.

this works as expected, so i thought my interpreter worked fine, but when I tested with a few variants of the hello world code, strange things happened.

this bf code should also print a hello world, but instead it prints out ²♣■■ÖFu ÖÖ■♦u

--<-<<+[+[<+>--->->->-<<<]>]<<--.<++++++.<<-..<<.<+.>>.>>.<<<.+++.>>.>>-.<<<+.

this bf code should also print a hello world, but instead the program gets stuck

+[-->-[>>+>-----<<]<--<---]>-.>>>+.>>..+++[.>]<<<<.+++.------.<<-.>>>>+.

this is the code I wrote to interprete brainfuck:

#include <stdio.h>

int main(int argc, char const *argv[])
{

    if (argc == 1)
    {
        printf("You must specify a file path\n");
        return -1;
    }

    //amount of memory locations available
    int mem = 30000;
    //creating an integer array with mem positions
    char arr[mem];
    //current memory position
    int index = 0;

    //setting everything to 0
    for (int i = 0; i < mem; i++)
    {
        arr[i] = 0;
    }

    FILE *file = fopen(argv[1], "r");

    if (file == NULL)
    {
        printf("ERROR, file couldn't be read\n");
        return -1;
    }

    //reading util the END OF THE FILE
    char c;
    while ((c = fgetc(file)) != EOF)
    {
        if (c == '+')
        {
            arr[index]++;
        }
        else if (c == '-')
        {
            arr[index]--;
        }
        else if (c == '>')
        {
            index++;
            index %= mem;
        }
        else if (c == '<')
        {
            index--;
            index %= mem;
        }
        else if (c == '.')
        {
            printf("%c", arr[index]);
        }
        else if (c == ',')
        {
            scanf("%c", &arr[index]);
        }
        else if (c == '[')
        {
            char temp = fgetc(file);
            int skip = 0;

            while (temp != ']' || skip != 0)
            {
                if (temp == '[')
                    skip++;
                if (temp == ']' && skip > 0)
                    skip--;

                temp = fgetc(file);
            }

            fseek(file, -1, SEEK_CUR);
        }
        else if (c == ']')
        {
            if (arr[index] != 0)
            {
                fseek(file, -2, SEEK_CUR);
                char temp = fgetc(file);

                int skip = 0;

                while (temp != '[' || skip != 0)
                {
                    if (temp == ']')
                        skip++;
                    if (temp == '[' && skip > 0)
                        skip--;

                    fseek(file, -2, SEEK_CUR);
                    temp = fgetc(file);
                }
            }
            else
            {
                continue;
            }
        }
    }

    fclose(file);
    return 0;
}

Thanks a lot if you can help me out on this one.

There's could be a problem with this piece of code, when index become negative.

        index--;
        index %= mem;

% operator retains the sign of the left argument, so -1 % mem is –1, not mem–1 you may expect.