Both of your attempts have limitations:

• If the double value is outside the range of the int type, converting to int is implementation defined.
• If the double value is negative but integral, returning (int)num - 1 is incorrect.

Here is an (almost) portable version that tries to handle all cases:

double my_floor_2(double num) {
    if (num >= LLONG_MAX || num <= LLONG_MIN || num != num) {
        /* handle large values, infinities and nan */
        return num;
    }
    long long n = (long long)num;
    double d = (double)n;
    if (d == num || num >= 0)
        return d;
    else
        return d - 1;
}

It should be correct if type long long has more value bits than type double, which is the case on most modern systems.

Both of your attempts have limitations:

• If the double value is outside the range of the int type, converting to int is implementation defined.
• If the double value is negative but integral, returning (int)num - 1 is incorrect.

Here is an (almost) portable version that tries to handle all cases:

double my_floor_2(double num) {
    if (num >= LLONG_MAX || num <= LLONG_MIN || num != num) {
        /* handle large values, infinities and nan */
        return num;
    }
    long long n = (long long)num;
    double d = (double)n;
    if (d == num || num >= 0)
        return d;
    else
        return d - 1;
}

It should be correct if type long long has more value bits than type double, which is the case on most modern systems.