What is deadlock? What are necessary conditions for deadlock to occur? code example

Example: what is deadlock

Deadlock is a situation where a set of processes 
are blocked because each process is holding a resource 
and waiting for another resource acquired by some other
process.Process 1 is holding Resource 1 and waiting for
resource 2 which is acquired by process 2, and process 2
is waiting for resource 1.