Unless explicitly declared static, an ordinary function is implicitly declared extern in C and C++. For example:
extern void func(int i); // extern is redundant
However, sometimes the extern qualifier is added to a function declaration to document the fact that it has external linkage and can be called from any module, or source file, for example:
// from <string.h>
extern size_t strlen(const char * s);
Do not confuse plain extern with extern "C"the latter has a special meaning, i.e., the identifier has C linkage rather than C++ linkage.