I want to give errors like this, while activating my plugin.
Which hooks should i use ? Can you share simple examples ?
In shortly,when user activate my plugin, is a directory's CHMOD isn't 777 , i'll warn it and stop activation.
Check out : WP Error
And this link covers how the plugin activation error works: http://www.squarepenguin.com/wordpress/?p=6
And I don't see any reason why you should ever chmod a directory to 777