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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 876
SEC. 10.8
ANDROID
845
Starting Processes
In order to launch new processes, the activity manager must communicate with
the
zygote
.
When the activity manager first starts, it creates a dedicated socket
with
zygote
, through which it sends a command when it needs to start a process.
The command primarily describes the sandbox to be created: the UID that the new
process should run as and any other security restrictions that will apply to it.
Zygote
thus must run as root: when it forks, it does the appropriate setup for the
UID it will run as, finally dropping root privileges and changing the process to the
desired UID.
Recall in our previous discussion about Android applications that the activity
manager maintains dynamic information about the execution of activities (in
Fig. 10-52), services (Fig. 10-57), broadcasts (to receivers as in Fig. 10-60), and
content providers (Fig. 10-61). It uses this information to drive the creation and
management of application processes.
For example, when the application launcher
calls in to the system with a new intent to start an activity as we saw in Fig. 10-52,
it is the activity manager that is responsible for making that new application run.
The flow for starting an activity in a new process is shown in Fig. 10-67. The
details of each step in the illustration are:
1.
Some existing process (such as the app launcher) calls in to the activ-
ity manager with an intent describing the new activity it would like to
have started.
2.
Activity manager asks the package manager to resolve the intent to an
explicit component.
3. Activity manager determines that the application’s process is not al-
ready running, and then asks
zygote
for a new process of the ap-
propriate UID.
4.
Zygote
performs a
fork
, creating a new process that is a clone of itself,
drops privileges and sets its UID appropriately for the application’s
sandbox, and finishes initialization of Dalvik in that process so that
the Java runtime is fully executing. For example, it must start threads
like the garbage collector after it forks.
5. The new process, now a clone of
zygote
with the Java environment
fully up and running, calls back to the activity manager, asking
‘‘What am I supposed to do?’’
6.
Activity manager returns back the full information about the applica-
tion it is starting, such as where to find its code.
7.
New process loads the code for the application being run.
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