IPC 类型

* Shared memory permits processes to communicate by simply reading and
writing to a specified memory location.
* Mapped memory is similar to shared memory, except that it is associated with a
file in the filesystem.
* Pipes permit sequential communication from one process to a related process.
* FIFOs are similar to pipes, except that unrelated processes can communicate
because the pipe is given a name in the filesystem.
* Sockets support communication between unrelated processes even on different
computers.