"waiting for target device to come online" in Android Studio 2.3

I was also having the same issue 2 days ago when i update my android studio, today I solve my issue when I was playing around with settings then I saw that in my SDK tools setting Android Emulator is uncheck so I simply checked that box and now emulator is working fine.

Try this steps may work for you also:

1. Go to SDK tools > SDK Tools
2. Check Android Emulator and click Apply

There are changes, though it's installed, you might need to update

I already have Emulator installed. But still had the same problem this morning with Ubuntu 16.04 and Android Studio 2.3,Below is what I did and it worked.

Double Check to see if these are available:

    sudo apt-get install lib64stdc++6:i386    
    sudo apt-get install mesa-utils

1. Locate the lib64 folder. On Ubuntu 64 bit, its located under ~/Android/Sdk/emulator/lib64.

2. In terminal, Navigate to the ~/Android/Sdk/emulator/lib64 folder ,then run the following commands:

move the libstdc++ file inside the folder libstdc++.bak and provide softlink

mv libstdc++/ libstdc++.bak
ln -s /usr/lib64/libstdc++.so.6 libstdc++

None of these solutions worked for me. After removing and installing the emulator from scratch several times, and also SDK Tools, I saw under developer options in the emulator that USB debugging was not active.

• Settings
• About Emulator Device
• Click several times on Build Number until it says that you are a developer
• Go back
• Get into developer options
• Put a check on "USB debugging"

if you docker is running you should close it.