[CMake] undefined reference to error when use "c" extension

[Greg Marr]

If you are including the .h file for the .c file in your .cpp file and it doesn't already have extern "C" in it, wrapping it like this when including it in your cpp should help:

extern "C"
{
#include "header.h"
}



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-------- Original message --------
From: J Decker <d3ck0r at gmail.com>
Date: 06/05/2015 10:33 PM (GMT-05:00)
To: Sunrise <helios.corona at gmail.com>
Cc: cmake at cmake.org
Subject: Re: [CMake] undefined reference to error when use "c" extension

c++ does name mangling on functions... so functions like 'f' become a much more complex name (as shown in the xxx not found in your error messages).
In order for C++ to not produce a mangled name C functions have to be defined as

extern "c" void f( void );
but 'extern "c"' is not liked by C... so you really need to define in the shared header something like...

#ifdef __cplusplus
#define CEXTERN extern "C"
#ese
#define CEXTERN
#endif

CEXTERN void f( void );

But of course since you don't know about name mangling I guess you don't know proper header usage either.  This is not a cmake issue, but a general C++ issue... and you'd do better asking stack exchange or something.

On Fri, Jun 5, 2015 at 6:37 PM, Sunrise <helios.corona at gmail.com<mailto:helios.corona at gmail.com>> wrote:
Hello,

I am linking my code to a library. My code is in C++ but the library is in C.

The problem is that whenever the extension of library implementations are "c" (not cpp), they are not linked and I get "undefined reference to" error.

Here is an example:

Suppose I have
./src/main.cpp   // as main file
./include/lib.h
./include/lib.c  // as a library

And the cmake file is

cmake_minimum_required(VERSION 2.8)
project(myproj)

set(INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include)
include_directories(${INCLUDE_DIR})
add_library(MY_LIB ${INCLUDE_DIR}/Lib.c)

set(EXECUTABLE_NAME "myproj")
set(SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src)
add_executable(myproj ${SOURCE_DIR}/main.cpp)

target_link_libraries(myproj MY_LIB)

This returns undefined reference to error, but if I rename lib.c to lib.cpp, everything works fine.

How can I resolve this? I do not want to rename the file to cpp, because there are a lot of library files and I prefer to keep the library implementations untouched.

Thanks.

--

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