Describe IPC

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@@ -103,3 +103,24 @@ level of efficiency is necessary for building the Kernel that most software
 programs depend on. The system call interface is exposed through man pages and
 header files. libc is the standard interface for making system calls into the
 kernel.
+
+# Define inter-process communication (IPC)
+
+> IPC refers specifically to the mechanisms an operating system provides to
+> allow the processes to manage shared data. - https://en.wikipedia.org/wiki/Inter-process_communication
+
+Operating systems provice IPC mechanisms to allow processes to communicate state
+with one another. This can be for many different reasons including taking a lock
+on a resource, streaming data from one process to another or synchronizing
+changes from one process to another.
+
+Some examples of IPC mechanisms are:
+
+* file: A file can be used as a lock to ensure that only one process performs
+  a restricted action at a time. `man flock`
+* signal: using the `kill` program one program can send a signal to another
+  program for various reasons such as reloading configuration or performing a
+  graceful shutdown.
+* socket: A TCP/UDP can be used for streaming data across a network.
+* UNIX domain socket: Can be used to stream data from one process to another on a host.
+* Pipes: A program can write to stdout which can be piped into stdin of another process.