why does it not concatenate name into greeting at the end?
int main(int argc, char **argv) {
    if (argc != 3) {
        fprintf(stderr, "Usage: greeting message name\n");
        return 1;
    }
    char greeting[20];
    char *name = argv[2];

    // Your code goes here
    strcat(argv[1], " ");
    int len = strlen(argv[1]);
    int space_left = 19 - len;

    greeting[0] = '\0';
    strncat(greeting, argv[1], 19);

    strncat(greeting, name, space_left);

    printf("%s\n", greeting);
    return 0;
}