Can't use -flto with -std=c99 in C program

Question

My application builds fine with -flto, but only if I do not also specify -std=c99.

If someone can help me, that would be wonderful. I have created a very simple test, below, to demonstrate the problem.

main.c:

#include "foo.h"
void main(int argc, char** argv) {
 int input = atoi(argv[1]);
 printf("%dn", foo(input));
}

foo.h:

inline int foo(int x);

foo.c:

#include "foo.h"
inline int foo(int x) {
 while (x < 900) {
 x += x;
 }
 return x;
}

Makefile:

CFLAGS += -flto -std=c99
LDFLAGS += -flto -std=c99

main : main.o foo.o
main.o : main.c foo.h
foo.o : foo.c foo.h

.PHONY : clean

clean :
 $(RM) main main.o foo.o

Results of running make:

cc -flto -std=c99 -c -o main.o main.c

In file included from main.c:3:0:

foo.h:1:12: warning: inline function  foo  declared but never defined [enabled by default]
 inline int foo(int x);
 ^
foo.h:1:12: warning: inline function  foo  declared but never defined [enabled by default]
cc -flto -std=c99 -c -o foo.o foo.c
cc -flto -std=c99 main.o foo.o -o main
/tmp/ccTDIBGZ.ltrans0.ltrans.o:ccTDIBGZ.ltrans0.o:function main: error: undefined reference to 'foo'
collect2: error: ld returned 1 exit status
make: *** [main] Error 1

Without the -std=c99 flags, make runs successfully and without warnings.

Answer 1

I don't think your program is valid C99 code (even ignoring the fact your main() has the wrong return type).

If a C99 function is declared inline (and not extern) then it is an inline definition, and will not be used for calls to that function from other translation units, so you need to declare it as extern or define it in every translation unit that calls it.

GNU C90 inline functions have different semantics to C99 inline functions:

"When an inline function is not static, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion. "