C: extern keyword

declaration (no memory allocation)
definition (memory allocated)


int foo(int a, int b); /* declare, by default compiler prepends extern to such declarations */

int foo(int a, int b) {
/* defined*/
}

int foo(int a, int b); = extern int foo(int a, int b)


int bar; /* declare and define; mem allocated */

extern int bar; /* declare; mem not allocated */

extern int bar = 0; /* declare and intialized; hence, mem allocated */


cf: One Definition Rule in C, C++

extern "C" is a linkage specification.