Is this function portable among all versions

I heard that the function ioctl() is not compatible or portable with all versions of UNIX. Is this true? If so why is it so?

The function ioctl() as you heard is not portable with all versions of UNIX. One of the reasons according to me could be because they perform device specific functions. If I am wrong someone suggest a better reason why is it so?

Yes as suggested by Timmy the function ioctl() is not portable. This is because by its inherent nature of non portability and also as said by Timmy because they perform device specific functions so if this need to be used with any other devise proper change in code in needed.