What makes a system property persistent ?

[FrankG]

Dear all,

I can set a system property via set_prop i.e. a adb shell and can ask
for this property via get_prop.

But what makes a property persistent ?

Some of the persistent properties are introduced via the Variable
ADDITIONAL_DEFAULT_PROPERTIES in
an core/main.mk , but no idea who uses this Variáble and what happens
with them.

Some are set by default.prop, but what is the story behind them ?

I cannot simply change default.prop, as it seem the be rewritten after
reboot.

So how make my own persistent property ?

Thanks a lot !

Frank

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[Mike Lockwood]

System properties are automatically persistent across reboots if their
name begins with "persist." But it sounds like you are just having
trouble setting the system properties due to permission problems.
Most system properties can only be written by root, while some can be
written by certain users like system (for the system server process)
or radio (for the RIL or phone app). In general you can not write
system properties unless you are a privileged user.

Mike

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Mike Lockwood
Google android team

[FrankG]

Thanks a lot Mike, for your fast answer.

I need this for a vendor extension, so I can modify
also part of the platform themselve and can also be
privileged user.

What I need is a) introduce a new persistent variable
and b) able to change them from a java system service and
from a modified version of Settings-App.
Where do I declare that a certain system property exists or even
better to say,
where must I introduce my own system property ?

Thanks a lot !

Frank

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> Mike Lockwood
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[Mike Lockwood]

You don't need to declare the property anywhere in advance before
setting it. You can use default.prop to set an initial value in the
build if you want, but that is not a requirement. Otherwise, the
property is undefined until someone calls property_set to give it a
value.

By default only root can set a system property. But if you need to
set it from some other process, you can change the property_perms[]
table in system/core/init/property_service.c to allow another process
to write it. Or better yet, use one of the existing prefixes in the
table if it makes sense instead.

Mike

[FrankG]

Hello Mike,

Thanks a lot. Verry kind of you .

I think with your answer I have now the missing details.

Have a nice day ! Frank

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