Why does the C preprocessor interpret the word "linux" as the constant "1"?

Why does the C preprocessor interpret the word "linux" as the constant "1"?

c linux gcc c-preprocessor

linux

How do the likely/unlikely macros in the Linux kernel work and what is their benefit?

linux gcc linux-kernel likely-unlikely

linux

How to add a default include path for GCC in Linux?

linux gcc include environment-variables

linux

Multiple glibc libraries on a single host

linux gcc glibc

linux

Is errno thread-safe?

c linux multithreading gcc

linux

Build .so file from .c file using gcc command line

c linux gcc shared-libraries

linux

"/usr/bin/ld: cannot find -lz"

linux gcc linker

linux

Clang vs GCC for my Linux Development project

c++ c linux gcc clang

linux

Make install, but not to default directories?

linux gcc makefile autotools

linux

ld cannot find an existing library

linux gcc linker shared-libraries shared-objects

linux

How to print the ld(linker) search path

linux gcc linker ld

linux

What is the advantage of GCC's __builtin_expect in if else statements?

c linux gcc built-in

linux

Static link of shared library function in gcc

linux gcc

linux

How to compile a 32-bit binary on a 64-bit linux machine with gcc/cmake

linux gcc 64-bit cmake 32-bit

linux

Where is PATH_MAX defined in Linux?

c linux gcc include clang

linux