Put libarmnn.so in the correct path, in other words, in the path that the project can find.
The cause of this problem is that the dependent libarmnn.so cannot be found when running./mnist_caffe. libarmnn.so is actually a compiled file in a directory called /home/lgg/armnnsdk.armnn.
There are two common ways to solve the problem. One is to run sudo gedit /etc/ld.so.conf, and then add the directory where libarmnn.so is located, /home/lgg/armnnsdk/armnn. Then run sudo /sbin/ldconfig for the configuration to take effect. However, this is generally not done for self-compiled generated files. Self-compiled generated libraries generally use the Method 2 configuration path. Second, run sudo gedit ~/.bash_profile and add export LD_LIBRARY_PATH=/home/lg/armnnSDK/armnn:$LD_LIBRARY_PATH. Or export LD_LIBRARY_PATH=/home/lg/armnnSDK/armnn:$LD_LIBRARY_PATH if it takes effect only on the current terminal.