Using strcat inside another function: Segmentation fault (core dumped)

Question

    static void foo(unsigned char *cmd)
    {
        strcat(cmd, "\r\n");
        printf("\r\nfoo: #%s#", cmd);

    }
    int main()
    {
        foo("test");
        return 0;
    }

Compiler says Segmentation fault (core dumped) What is the actual problem here?

Answer 1

You have undefined behaviour. You are not allowed to modify string literals. cmd points to a string literal and strcat() attempts concatenate to it, which is the problem.

  int main(void)
    {
        char arr[256] = "test";
        foo(arr);
        return 0;
    }

You generally need to be careful when using strcpy() and strcat() etc in C as there's a possibility that you could overflow the buffer. In my example, I used an array size of 256 which is more than enough for your example. But if you are concatenating something of unknown size, you need to be careful.

Answer 2

In C string literals (like "test") are read-only arrays of characters. As they are read-only they can't be modified. As they are arrays they have a fixed size. You both try to modify the string literal, and extend it.