Operating Systems

A high-level programming language built in the C programming language.C used to construct the first operating system, which was UNIX. Later on, the C programming language was used to write Microsoft Windows and several Android apps. With the help of C language You can write your own operating system.
Windows Kernel, Linux Kernel and Apple’s OS X kernel are mostly written in C Language.

Graphical User Interface(GUI)

GUI: Stands for "Graphical User Interface" and is pronounced "gooey." It is a user interface that includes graphical elements, such as windows, icons and buttons. The term was created in the 1970s to distinguish graphical interfaces from text-based ones, such as command line interfaces. However, today nearly all digital interfaces are GUIs.
Adobe Photoshop has been one of the most widely used picture editors. It was created entirely with the aid of the C programming language. Furthermore, C was used to develop Adobe Illustrator and Adobe Premiere.
Today, each OS has its own GUI (such as Ubuntu for Linux). Software applications use these and add additional GUIs of their own, for example Microsoft Word and Excel, or Adobe Photoshop. All internet browsers, such as Chrome, Internet Explorer and Firefox use their own GUIs to allow the user to navigate through websites which may also have their own GUIs (such as Facebook, Instagram or WordPress sites).
If a user opens, for example, a video from a streaming video player inside a website, they will interact with four different GUIs in total:
The OS’s
The browser’s
The website’s
The video player’s

Embeded System

An embedded system is a combination of computer hardware and software designed for a specific function. Embedded systems may also function within a larger system. The systems can be programmable or have a fixed functionality.
C is the most popular forprogramming language in software field for developing electronic gadgets. C provides optimized machine instructions for the given input, which increases the performance of the embedded system. Most of the high-level languages rely on libraries, hence they require more memory which is a major challenge in embedded systems.

Modern Programs

Git is a very important application written in C. The Linux Operation System is written in C. Most applications where memory management combined with performance is important are written in C or C++ (MongoDB is written in C++ as is Nodejs). Javascript engines such as V8 (C++) and Spidermonkey (C). The Python and Ruby programming languages are written in C. The Gimp Toolkit is a cross platform C gui library. There are native C windowing libraries for Linux (XLib) and MS Windows. There are many cross platform C++ windowing libraries. QT is very good. GUI development lends itself well to OO so even C libraries like GTK tend to use that paradigm.
Since its creation, C has become one of the most widely used programming languages in the world. Well-written C programs are efficient. The language is more flexible than other languages: It can work at the highest levels

Assemblers/compiler,interpreter

You can widely use the C programming language to develop compilers, one of its most popular applications. Many other languages’ compilers were created with the connection between C and low-level languages in mind, making it easier for the machine to grasp what was being written. Many prominent compilers, such as Clang C, Bloodshed Dev-C, Apple C, and MINGW, were developed with the C programming language.

Databases

Most of the RDBMS and non RDBMS databases are Developed in C language.
MySQL: is open-source project that is used in relational database management systems (RDBMS). It was developed in the C/C++ programming language
Oracle:It is a database commonly used for running online transaction processing (OLTP), data warehousing (DW) and mixed (OLTP & DW) database workloads. It is multi-model type database.Oracle Database is available by several service providers on-prem, on-cloud, or as a hybrid cloud installation.it is developed by Oracle corporation and Written in C,C++ and Assembly.

Network Drivers

Network programming enables processes to communicate with each other over a computer network, but it is a complex task that requires programming with multiple libraries and protocols. With its support for third-party libraries and structured documentation, C is an ideal language to write network programs.

Drivers

Another application of C is to write driver softwares like Keyboard driver, Network driver, mouse driver etc.
Driver: A computer driver is a set of files that help your computer's operating system interface with certain pieces of hardware, like a graphics card. Think of it as a team of translators passing instructions between a factory's management and its machinery operators.

MicroController

A microcontroller is a small and low-cost microcomputer, which is designed to perform the specific tasks of embedded systems like displaying microwave’s information, receiving remote signals, etc.
A microcontroller also called MCUs or Microcontroller Unit is a single integrated circuit (IC) that is used for a specific application and designed to implement certain tasks.Products and devices that have been automatically controlled in certain situations, like appliances, power tools, automobile engine control systems, medical equipment, high-end consumer electronics, rugged industrial devices, and computers are great examples, but microcontrollers reach much higher than these applications.In daily life, we use different embedded systems like coffee machines, microwaves, climate control systems etc. These all are mostly programmed in C.

Print Spooler

A printer spooler is a simple program that manages all print jobs that are submitted to the print server or a computer printer.Nowadays, the most common use of spooling is printing: documents formatted for printing are stored in a queue at the speed of the computer, then retrieved and printed at the speed of the printer. Multiple processes can write documents to the spool without waiting, and can then perform other tasks, while the "spooler" process operates the printer.these are programmed in C

Text Editors

A text editor refers to any form of computer program that enables users to create, change, edit, open and view plain text files. They come already installed on most operating systems but their dominant application has evolved from notetaking and creating documents to crafting complex code