Unexpected undefined reference - c++

Question

I'm getting an undefined referenced error, not knowing the reason why.
So I have 2 files which makes a static lib : keyboard_input.c, keyboard_input.h
Here's the content of the .h file:
#ifndef __MOD_KBINPUT__
#define __MOD_KBINPUT__
int kbInit();
int kbWait();
int kbTest();
#endif
And the CMakeLists.txt file looks like this:
FILE(
GLOB_RECURSE
sources
*.c
)
INCLUDE_DIRECTORIES("${PROJECT_SOURCE_DIR}/include/utils/kbreader")
ADD_LIBRARY(keyboardReader ${sources})
Compiling this lib gives some warnings:
src/utils/kbreader/keyboard_input.c: In function ‘kbInit’:
src/utils/kbreader/keyboard_input.c:13:14: warning: assignment from incompatible pointer type [enabled by default]
src/utils/kbreader/keyboard_input.c: In function ‘kbWait’:
src/utils/kbreader/keyboard_input.c:21:55: warning: passing argument 4 of ‘fread’ from incompatible pointer type [enabled by default]
/usr/include/stdio.h:708:15: note: expected ‘struct FILE * __restrict__’ but argument is of type ‘struct FILE *’
Now, for my main executable (main.cpp):
#include <keyboard_input.h>
int main()
{
kbTest();
return 0;
}
Processed by the following CMakeLists.txt file:
include_directories("${PROJECT_SOURCE_DIR}/include/utils/kbreader")
file(
GLOB_RECURSE
srcs
*.cpp
)
add_executable(
PEM
${srcs}
)
target_link_libraries(PEM keyboardReader)
Ends up getting that error:
CMakeFiles/PEM.dir/main.cpp.o: In function `main':
main.cpp:(.text+0xb): undefined reference to `kbTest()'
collect2: ld returned 1 exit status
make[2]: *** [src/PEM/main2/PEM] Error 1
make[1]: *** [src/PEM/main2/CMakeFiles/PEM.dir/all] Error 2
The libkeyboardReader.a is created, and the kbTest() function doesn't do anything except
{return 0; }
If I set the definition of kbTest() in the header file, it works.
But there's something i don't get, when i type: make keyboardReader here is the output:
[ 73%] Building C object src/utils/kbreader/CMakeFiles/KeyboardReader.dir/keyboard_input.c.o
[Warning explained above]
Linking C static library ../../../lib/libKeyboardReader.a
Is there something wrong? Does the note error message makes my lib omit the keyboard_input.c file?

Answer 1

You're mixing C and C++ files. To make that work, you just have to tell the C++ compiler that it's calling a C function, by changing the header file like so:
#ifndef MOD_KBINPUT
#define MOD_KBINPUT
/* note I also fixed the macro so you aren't using a system-reserved name */
#if __cplusplus
/* this is the important part */
extern "C" {
#endif
int kbInit();
int kbWait();
int kbTest();
#if __cplusplus
}
#endif
#endif
Otherwise the C++ compiler assumes the function will be given a C++ internal name (which encodes all the type information in the signature, this is what lets the linker distinguish between overloaded functions) and then the linker doesn't find it.