Examples may include the usage of:

• semaphores;
• shared memory;
• timeouts and alarms;
• signals;
• pipes;
• files;
• sockets;
• ...
• formatted input and output;
• memory allocation and deallocation;
• sorting;
• ...

From ZBXNEXT-3353 (1):

...we should provide an example of synchronization in module developer documentation.

Currently in dummy module we call srand() in zbx_module_init() which is called by Zabbix main process before forking. This means that all poller processes will give out the same sequence of pseudo random numbers when polling dummy.random[]. Sorting this issue can be a good ground for an example.