the.com/extern
A variable that swears it lives elsewhere, and trusts you to keep that promise.
means A C keyword declaring that a variable or function is defined in another file, not here.
from From Latin externus, meaning outward or foreign — the same root that gave us external and the awkward dinner guest who isn't family.
Declares, never definesPromises existence without allocating any memory.
Linker's jobThe linker hunts the real definition at build time.
Default for functionsFunctions are implicitly extern unless marked static.