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Glossary

Assembly language​

An assembly language is a low-level programming language which is processor-specific and has a direct correspondence between its instructions and those available on said processor.

Automatic memory​

Automatic memory is a form of memory which is stored on the stack. This type of memory allocation is automatic and its lifetime is the duration of the scope it is allocated in. Hence, there is no need to explicitly release the allocated memory.

Command line interface​

A command line interface (CLI) is a user interface where interaction between the user and the computer is done textually. For example, when you interact with your shell in a terminal, you are using a CLI.

Compiler​

A compiler is a program that translates code written in a source language into an equivalent code in a target language. Generally, compilers translate code from a high-level programming language into a low-level language which will then be used to create an executable file.

Dynamic memory​

Dynamic memory is a form of memory which is stored on the heap. This type of memory is usually allocated by calling functions from the malloc family in the standard library. Its lifetime is that of the program, until it is released by calling free. A program exiting without releasing all of its dynamic memory is said to be leaking.

File descriptor​

A file descriptor is an integer that uniquely identifies an open file. When a process is started, 3 file descriptors are automatically opened by the kernel: stdin, stdout and stderr.

Graphical user interface​

A graphical user interface (GUI) is a user interface that includes graphical elements such as windows, icons, buttons... GUIs can be seen as the counterpart of command line interfaces and are generally manipulated using a keyboard and mouse.

Hardware​

Hardware is the name given to all the physical parts of a computer.

High-level​

As opposed to low-level, it is a technical term describing components with a high level of abstraction from a computer's inner workings. We often describe programming languages with this term, for example Java and Python are considered high-level languages.

Heap​

The heap is the portion of memory dedicated to dynamic allocation. Unlike the stack, the heap can be fragmented into many blocks of various sizes. Allocating on the heap is slower than on the stack because you do not know the size in advance and needs to find an empty spot. Data allocated on the heap must be freed manually because its lifetime linked to the one of the current process.

Kernel​

A kernel is the core of an operating system, with a complete control over the system. Linux is an example of a free and open-source kernel, whereas GNU/Linux is considered to be the whole operating system containing the Linux kernel as well as various tools from the GNU system.

Linker​

A linker is a program which takes several object files to combine them into a single binary file. The output can be either an executable, a library, or even another object file. Linkers are often used at the end of the compilation toolchain, to combine files generated by the assembler.

Low-level​

As opposed to high-level, it is a technical term describing components with little or no abstraction from a computer's inner workings. For example, C and C++ are considered low-level programming languages.

Multitasking​

Multitasking is the execution of multiple tasks in a concurrent manner. This does not necessarily mean that tasks are computed in parallel, but rather that many tasks can be interrupted and split into parts to share the computer resources over a period of time.

OS​

An operating system (OS) is a software program which manages both hardware and software resources to allow user interaction. Some examples of common operating systems are: Windows, macOS and GNU/Linux.

Process​

A process is the set of resources used to execute a program (its code, memory, open files, ...). Inside the process, the execution is actually performed by one or multiple threads.

Shell​

A shell is a computer program serving as an interface with the operating system. The English word shell was chosen because it acts as a shell around the kernel. Generally, users interact with a shell either using a command-line interface (CLI) or a graphical user interface (GUI).

Stack​

The stack is a portion of memory dedicated to automatic memory allocation. It is mostly used to store local variables and function parameters. Unlike the heap, allocation on the stack is very fast because there is no fragmentation (the next empty spot is always the top of the stack), and the size is known in advance. Variables allocated on the stack are automatically freed when they go out of scope.

Static memory​

Static memory is allocated when the program starts. Its size is fixed when the program is created and its lifetime is that of the entire program. It is generally used for global variables and variables created with the static keyword. These variables are only initialized the first time their scope is entered and hence preserve their previous value when a scope is entered again.

Stderr​

The standard error (stderr) file descriptor is an output file descriptor automatically opened by the kernel on process creation which is written to to inform the user about errors and warnings. This file descriptor's number will be 2.

Stdin​

The standard input (stdin) file descriptor is automatically opened by the kernel on process creation and represents input. The input for the process is read from it. For example, if you pipe the output of a process A into a process B, you are directing the standard output of process A into the standard input of process B. This file descriptor's number will be 0.

Stdout​

The standard output (stdout) file descriptor is an output file descriptor automatically opened by the kernel on process creation which is written to for normal output messages. This file descriptor's number will be 1.

Stream​

A stream is a sequence of data made available to a program over time for it to be processed.

Syscall​

A system call (syscall) is the way in which a program requests a service from the kernel. For example, a regular program cannot interact directly with a disk so it must use the syscall API exposed by the kernel to manipulate files on a disk (open, read, write, close). The man section containing information about system calls is the second one.

Terminal​

A terminal is the interface allowing access to the command line. A terminal can be seen as the "container" program which encapsulates our shell and allows us to interact with it.