In-built lock. Its already there on your zip. Just check it.
Most of the jeans zips now come with In-built lock. When you zip just make sure that zip-hooker is placed vertically Up/DownDown (Not in between, thisthis will lock the zip.) Just try to open the zip with hook placed Up/Downdown. It won't open Just like below image.(Zip hook placed vertically down here.)
If you zip and don't keep hooker either up/downdown zip will eventually run down due to strain on it.
Visit YKK website for more info. If you see zips anywhere, there's 70% chance that it's made by YKK.
Some low quality jeans might not use YKK produced Zip. That's why you should always check YKK mark on zip before buying jeans.