The critical software infrastructure written in C. Virtually every layer of modern computing depends on C code.
Every major OS kernel is written in C. Linux (30M+ lines), Windows NT kernel, macOS/iOS (XNU/Darwin), FreeBSD, OpenBSD, Android (Linux kernel), Minix, QNX, Zephyr RTOS. The OS is where C lives.
The world's data sits in C. PostgreSQL, MySQL, SQLite, Redis, MongoDB (core engine), memcached, LevelDB. When nanoseconds per query matter, C is the only option.
The tools that build software are built in C. GCC, Clang/LLVM (C/C++), CPython, Ruby (MRI), Lua, PHP (Zend engine), Perl. C compiles the world's code, including itself.
The internet runs on C. nginx, Apache httpd, curl, OpenSSL/LibreSSL, dnsmasq, Bind, iptables/nftables, Wireshark, tcpdump, OpenVPN, WireGuard. Every packet on the internet is handled by C code.
From pacemakers to Mars rovers. C is the language of embedded: microcontrollers (ARM Cortex-M, AVR, PIC, MSP430), IoT devices, automotive ECUs, industrial PLCs, medical devices, avionics. When you have 4KB of RAM, C is your only option.
Performance-critical game infrastructure: id Tech engines (Quake, Doom), raylib, SDL, OpenGL/Vulkan implementations, physics engines. C provides the frame-rate-critical inner loops.
Crypto must be fast and constant-time. OpenSSL, libsodium, GnuPG, OpenSSH, Linux crypto subsystem, hardware security modules. Timing-safe C is the foundation of digital security.
BLAS, LAPACK, FFTW, GNU Scientific Library, HDF5, NetCDF. The numerical libraries that power physics simulations, climate models, and machine learning backends are written in C (and Fortran).
The Mars rovers (Curiosity, Perseverance) run C. The Voyager probes run C. Satellite firmware, flight control systems (DO-178C certified), and spacecraft telemetry are all C. When rebooting isn't an option, C is the choice.
Pacemakers, insulin pumps, MRI controllers, ventilators, patient monitoring systems. FDA-regulated medical software requires deterministic timing and memory behavior that only C (and sometimes Ada) can guarantee.
A modern car has 100+ ECUs running C code. Engine management, ABS, airbags, infotainment, ADAS (advanced driver assistance). MISRA C is the automotive safety standard for C. AUTOSAR runs on C.
High-frequency trading systems where microseconds = millions of dollars. Exchange matching engines, risk systems, market data feeds. When latency is measured in nanoseconds, managed languages are too slow.