Does widrawal is always safe in terms of trying to not to get pregnant?

Not at all: To be honest, this is a terrible form of contraception as it is very difficult to appropriately time "withdrawal". Additionally, while precum doesn't have a lot of sperm in it, there still some and it's possible to become pregnant from just that. Please really consider another form of birth control and be aware that the "pull out" method does not protect your from sexually transmitted diseases.

Onanism: the pull out method, or onanism, is definitely not a fool proof guarantee. Contraception, abstinence or timing with menses will improve chances of not getting pregnant if not desired.