It is easy to assume C is a relic that only matters for history lessons, but the opposite is true: C runs quietly underneath nearly every piece of technology you touch every day. Your phone's operating system, your laptop's firmware, your car's engine controller, and the database behind your favorite app almost certainly have C somewhere in their foundations.

Operating system kernels need to manage memory, schedule processes, and talk directly to hardware with as little overhead as possible — exactly the job C was designed for. The Linux kernel, which powers everything from Android phones to web servers to supercomputers, is written almost entirely in C. The low-level core of Windows NT and the XNU kernel at the heart of macOS and iOS also rely heavily on C (with some C++ layered on top). Most other production operating system kernels in the world are written primarily in C.

Embedded devices — microwave ovens, washing machines, thermostats, car engine control units, medical devices, industrial controllers — typically have only a few kilobytes of memory and no operating system at all. There is no room for a garbage-collected runtime or a virtual machine. C compiles directly to compact machine code that can run on hardware this constrained, which is why it remains the default language for embedded firmware and microcontroller programming to this day.

A device driver is the piece of software that lets an operating system communicate with a specific piece of hardware — a graphics card, a network adapter, a printer. Because the kernel itself is written in C, drivers are written in C too, so they can plug directly into the kernel's data structures and calling conventions without any translation layer in between.

Databases need to move enormous amounts of data as efficiently as possible, which makes C (and C++) the natural choice for database engines. SQLite — almost certainly the most widely deployed database engine in the world, embedded in browsers, phones, and countless apps — is written in C. Large parts of PostgreSQL and MySQL's server internals are written in C as well.

Higher-level languages need something to actually execute their code, and that something is very often written in C. CPython, the standard implementation of Python, is written in C. Ruby's original and still most widely used implementation, MRI (Matz's Ruby Interpreter), is also written in C. This is why both languages offer a "C extension" mechanism for writing performance-critical code.

Aerospace systems, automotive control software, medical devices, and industrial automation all demand predictable, bounded execution time — a hard requirement that C's lack of hidden runtime behavior (no garbage collection pauses, no unpredictable allocations) makes it much easier to satisfy than in languages with heavier runtimes.