What is the use of extern inline?

[Shriramana Sharma <samjnaa@gmail.com>]

In man:gcc under -fkeep-inline-functions I first came across the usage 
of "extern inline". For normal functions, extern declares that the 
function exists elsewhere. But this does not seem to work for inline 
functions:

lib.cpp:
---------------------------------------
inline int foo ( void ) { return 2 ; }
---------------------------------------

main.cpp:
---------------------------------------
# include <cstdio>
extern inline int foo ( void ) ;
int main ( void ) {
	printf ( "%d\n", foo () ) ;
}
---------------------------------------

$ g++ -c main.cpp lib.cpp
main.cpp:3: warning: inline function ‘int foo()’ used but never defined
$ g++ -o main main.o lib.o
main.o: In function `main':
main.cpp:(.text+0x12): undefined reference to `foo()'
collect2: ld returned 1 exit status
$

nm on lib.o returned absolutely nothing! Apparently code for an inline 
function is not produced if it is never called. If it is called, and it 
is not labeled static inline, then it is both inlined and compiled 
separately. Strange, but ok.

So I tried adding a dummy function to lib.cpp calling foo() just to make 
it get compiled. Then compiling main.cpp gave the same warning as above, 
but linking and execution went on ok.

But I discovered that I don't need to have the inline keyword in the 
declaration of foo() in main.cpp. In fact, removing the inline keyword 
allows compilation without warning.

So what is the *unique* use of extern inline? I mean, where we cannot do 
without it?

Shriramana Sharma.

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