What's different between '-' use flag and not having at all

[whjeon]

Let's say some package has 'A' use flag.
If I don't have any use flag on make.conf or package.use, then A won't be compiled right?
then where is difference with '-A' flag in make.conf?

[NeddySeagoon]

whjeon,

Short answer ... maybe

Longer answer. USE flag settings are hierarchical, with lower level settings overriding higher level settings.

An the top level are the USE flags in your profile. These are set if there is no USE in make.conf.
USE in make.conf adds to or rake away from the profile USE settings, so USE=+foo sets a flag, USE=-foo unsets it.
The + is not required.

That's all the global settings.
The same is true in package.use and even on the command line.

You use USE=-foo to unset a USE flag that is inherited from further up the hierarchy.

Its not an error to set/unset a USE flag multiple times. Only the last setting takes effect.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

[whjeon]

[NeddySeagoon]

whjeon,

There is no difference between USE=foo not being set anywhere in the hierarchy to setting USE="-foo" somewhere.
All USE flags are off until they are turned on somewhere.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

[Tony0945]

whjeon,
Some packages have use flags that are automatically turned on unless they are turned off. These are indicated by (using your example) +A in the IUSE line in the ebuild. That flag will be used even if it is not set in the profile or make.conf. You have to positively put -A somewhere (I prefer package.use) to turn it off. If IUSE does not have a + sign in front it will not be turned on unless set by the profile , make.conf, or package.use.

[whjeon]