Develop Reference

How "adb reboot bootloader" works internally?

I am seeing an issue with my android images where the command adb reboot bootloader simply reboots back the android, instead of going to bootloader mode.
In order to fix the issue, I did some study and find that there are acually two things, adb and adbd and the host and target devices communicate using the TCP protocol over sockets.
So, the interesting thing is commands like adb shell and adb devices are working but not the reboot bootloader. I want to understand what the adbd on receiving the reboot bootloader. Does it change the boor order, sets some flag, changes EFI vars....?
Can you please point to some good links or understanding you can share?
PS : I am working on embedded device environ, similar to raspberry pi...

This is how adb reboot bootloader works on a standard Android device connected via USB (the only transport supported by the standard Android bootloader in the fastboot mode):
adb client sends the reboot bootloader command to the adb server (over TCP)
adb server forwards the reboot bootloader command to the adbd on the device (over USB)
adbd sets the sys.powerctl property to reboot,bootloader
sys.powerctl change triggers the init.rc rule which runs powerctl init's built-in
which does _NR_reboot syscall
which sets the reboot to bootloader flag and reboots the device
On the next power up the bootloader would see the flag and go to the fastboot mode. But only if USB is connected.

adb wifi often go offline, how to keep adb online?

I'm using Android tablet communicating with a USB device through USB port, the tablet only has one usb port, so I use adb wifi as my debug tool. However, when I connect adb through WIFI, the connection won't keep long(only a few minutes us usually), how to keep the adb wifi connection online all the time?
Another question, is it possible to use USB HUB to connect the tablet with both USB flash-drive and my computer, and keep adb running?
Many thanks!!!

After lots of unsuccessful attempts,I have noticed that turning Allow ADB debugging in charge only mode specification on , solved the problem . It makes the device online and accessable over wifi to ADB.
You can find it under Developer options section.

Here is what I found out from researching everywhere and developing on my own for more than a week.
Q1. adb service often crashes if it is not doing any executions for a long long time.
A1. calling adb commands time to time keeps adb server online.
Q2. wifi connected device sometimes disconnect.
A2. Before you check your wifi on the connected device, check the wifi that your ADB server is running on. Ping to google and if it is well connected, ping to the device.
Wireless can disconnect on its own and even wired connection is sometimes disconnected. so calling "adb connect ..." when the device is not already connected is necessary. You could do this easily by saving "adb devices > device.txt" and query out whether your expected wifi is on the list. If not, connect again..(do this in another thread).
Q3. wifi connected device is sometimes offline.
A3. This is because by TCP/IP connection your ADB server is now on Time_Wait stance. The last connection was interrupted that the next connection is now offline because the last connection is not completely shut down. You can either manually disconnect/connect your device's wifi or restart TCPIP connection by ex) "adb tcpip 5555"
If you want to do this automatically, you are going to have to disconnect/connect your device's wifi programmatically.
Leave any other questions and I can answer on the comments.

When connection is lost and adb devices shows device is offline, like this:
prompt> adb devices
List of devices attached
192.168.1.1:5555 offline
then adb disconnect IP followed by adb connect IP will often work:
prompt> adb disconnect 192.168.1.1
disconnected 192.168.1.1
prompt> adb devices
List of devices attached
prompt> adb connect 192.168.1.1
connected to 192.168.1.1
prompt> adb devices
List of devices attached
192.168.1.1:5555 device

Use adb over wi-fi without any third-party apps. Follow the steps.
Connect device via USB
Open your terminal and check your device is listed with adb devices command
Type adb tcpip 5555 and enter (or use another port instead of 5555)
Plug out your USB cable. You don't need it anymore.
Take your Android phone and go Settings/About/Status
Check the IP address part. Let's say your IP address is 192.168.x.y
In your terminal, write the command adb connect 192.168.x.y
Now, you have connected your device via wi-fi.
You can see your device in adb devices
Enjoy it! :)

This seems like an issue with your wifi. As a precaution, just keep the terminal open and whenever you need to test just press the 'UP arrow' key to get to previously ran adb connect "<ip>" command.

I tried many ways but this one worked out for me:
force the display on and of in a loop would keep the device online
adb shell input keyevent 224
timeout 2
adb shell input keyevent 26
timeout 58
this turns the display on every minute for 2 seconds

So, I'm using Scrcpy and Adb to wirelessly connect to my Phone(for developing Flutter Apps) and since the screen wasn't staying awake even by Scrcpy --stay-awake command.
I had to change Lock Settings=>Lock Screen => Sleep, and settings to Never Sleep.

adb disconnect <ip-address >,disconnected the same device on the same server using by two user

Two user connected same android device through putty.
After seen adb devices can seen device is connected to both the putty users.
Doing adb disconnect <ip-address> by one user on the same server by putty is disconnecting the other user putty devices too.
Example:
Both the user connected through linux server daddy.
Open putty both the user and connect the android device through ip address which is same.
Now if one user disconnect the android device by command adb disconnect <ip-address>, another user too disconnected.
run adb devices,can't see any device connected through both the user.

If different users have their different Android devices (different clients), you can make different user start their adb server in different ports to listen the connections. Then different users can control their clients without interfere with each others.
$ adb -P 5038 start-server
$ adb -P 5038 connect 192.168.1.100:7777
$ adb -P 5038 disconnect
If different users in your server connect to the same device (client), there is no way to let different users to control the device at the same time. The reason is the adb daemon on the device can only be allowed listen to one port when you start it with command adb tcpip 7777.

Android Host Mode USB debugging

I am developing an application where my android phone connects to a USB accessory. However, when the accessory is connected is there any easy way to debug the application?
Can we use a micro usb hub or similar device?

When debugging applications that use USB accessory or host features, you most likely will have USB hardware connected to your Android-powered device. This will prevent you from having an adb connection to the Android-powered device via USB. You can still access adb over a network connection. To enable adb over a network connection:
Connect the Android-powered device via USB to your computer.
From your SDK platform-tools/ directory, enter adb tcpip 5555 at the command prompt.
Enter adb connect <device-ip-address>:5555 You should now be connected to the Android- powered device and can issue the usual adb commands like adb logcat.
To set your device to listen on USB, enter adb usb.
source: Android developer site

You could write an app that reads the logcat output on the phone and displays it on the screen and/or saves it to a file which you can pull with adb afterwards.
The app must be given READ_LOGS permission, which you can do with adb shell pm grant com.package.appname android.permission.READ_LOGS (at least on a rooted phone).
Another alternative might be to log over WiFi. adb has an option to connect over TCP/IP, but this is not something I've tried so I don't know how difficult it is to set up or how well it works.
Unless your logs are really verbose it's sometimes enough to run through your USB accessory use-case and then unplug the accessory and connect the phone to your computer to catch the logcat output. The logs are buffered up to a certain amount, so you can get a reasonable amount of logs using this method.

You could run an Android image in Virtualbox. This would allow you to connect your USB device to the virtual device, and debug over the bridged network connection.
I haven't figured out how to convert the SDK .img OS images to VDIs for use in Virtualbox, but there are plenty of canned VDIs out there.

adb logcat on a real phone using tcp/ip

Can I dump logs from an android phone over wifi (using tcp/ip) using adb? adb connect can be used, but it needs some server to be running on the phone (at 5555 port). How do I start this server?
Or the only way to get logs from a phone is by connecting it as a USB device?

Install adbWireless on your phone. Run the application and click on the big button (you cannot miss it!). It will activate ADB over Wifi and display the URL to use to connect to it with the adb command.
On your computer, run the adb command with the connect parameter. The usage for adb says:
connect <host>[:<port>] - connect to a device via TCP/IP
Port 5555 is used by default if no port number is specified.
Obviously the computer and the smartphone must be on the same Wifi network.

zero4
All you are trying to do is drop adb logcat command on the device & send output stream to a remote location. I would suggest, read this post about how to run shell commands in your app.
The summary is
Run "adb logcat"
Collect Output Stream of the command in a file on device
& Finally post that file to your local server OR Manually pull that file from device
The post contains link to everything you are looking for.

Android is very paranoid when it comes to network access. Without root access, you can't really run any servers, just clients. In short, without root, look at the answer from 100rabh.
If you do have root, you could either open up your network stack for incoming connections on port 5555, or you could hack adb to do the inverse connection (that is, connect to your client). The latter is way more secure and shouldn't really be to hard to do. (I haven't looked at the code for a while, though.) The communication bits for all parts of adb is handled in one and the same library, for all three parts of adb (server, daemon and client).
By the way, what you refer to as a server on the phone is really the adb daemon.