Inter-process Communication

The operating system provides various methods for processes to communicate with each other, called IPC (inter-process communication). One obvious mechanism is through the file system – by writing and reading from the same files, processes can share data. Others include signals, semaphores, shared memory mapping, sockets, pipes, and so on.

We will discuss these mechanisms in more detail later in the course, but for now you should understand a few basic types of IPC: Signals, Pipes, and Sockets.

Signals

A process with appropriate privileges can request to send a signal to another process; the kernel will handle the delivery of that signal next time the target process returns from kernel mode, which typically happens so often that signals usually appear to arrive almost instantaneously. If a process is sleeping, the kernel will also “kick” a signaled process and add it to the scheduler queue so that it can be woken up and respond to the signal. Sometimes the kernel also generates signals directly, such as SIGSEGV, when a process does something it’s not supposed to.

Each type of signal has a different meaning and default behavior associated with it. A process can also register its own custom signal handlers for most signals in order to override the default behavior; SIGKILL and SIGSTOP are the two exceptions to this rule. Below is a list of some commonly encountered signals, along with their default behaviors.

Signal Name	Default Action	Description
SIGABRT	Core dump [1]	Abort signal
SIGALRM	Terminate	Timer signal
SIGCHLD	Ignore	Child stopped or terminated
SIGCONT	Continue	Continue (if stopped)
SIGFPE	Core dump	Floating-point exception
SIGHUP	Terminate	Hangup detected on controlling terminal or death of controlling process
SIGINT	Terminate	Interrupt from keyboard
SIGKILL	Terminate	Kill signal
SIGPIPE	Terminate	Broken pipe: write to pipe with no readers
SIGQUIT	Core dump	Quit from keyboard
SIGSEGV	Core dump	Invalid memory reference
SIGSTOP	Stop	Stop process
SIGTSTP	Stop	Stop typed at terminal
SIGTERM	Terminate	Termination signal
SIGTRAP	Core dump	Trace/breakpoint trap
SIGTTIN	Stop	Terminal input for background process
SIGTTOU	Stop	Terminal output for background process
SIGUSR1	Terminate	User-defined signal 1
SIGUSR2	Terminate	User-defined signal 2

[1]

A core dump is when a process’s memory and state is dumped to a file before it terminates, which can be opened in a debugger to investigate the cause of the issue.

Pipes

Processes can also open a special type of a file called a pipe; pipes are anonymous (i.e. they don’t exist as named files in the file system), and they are managed by the kernel as a simple FIFO (first-in, first-out) data buffer. Each pipe has a read and a write end, allowing one-way communication between two processes. Since pipes don’t exist in the file system, the two ends of a pipe become distributed to separate processes by inheriting the ends of a pipe that was created by a shared ancestor.

A pipe usually has just one writer and one reader, but there is no restriction on the number of readers or writers that have it open at a time. There is still only one copy of data in the pipe, so multiple readers and writers may interfere with each other unless designed to operate cooperatively. This type of arrangement is often used to implement parallel processing, such as job queues – multiple writers can add job descriptions to a queue, and multiple readers can consume whatever jobs are available.

Unix Sockets

Sockets are similar to pipes, except that they are associated with a named resource so that unrelated processes can access them. There are different types of sockets, which includes familiar socket types such as TCP/IP and UDP/IP sockets which operate over a network and are bound to URLs. Unix also provides a type of socket called a Unix Domain Socket, which is bound to a path in the file system; these are also called fifos or named pipes, and are typically used by system services running in the background to provide an access point for users to interact with and request service.