I´m currently writing a Go wrapper for C api which contains header with this ifdef:
#ifdef __cplusplus
#define TEST_INLINE inline
#else
#define TEST_INLINE
#endif
TEST_INLINE int callC_inline (){
return 1;
}
Unfortunately, I cannot change header since it's a third-party code. The code compiles fine if I pass -Wl,--allow-multiple-definition to linker, but I think it's a bad practice. So, I'm interested is there any flag I can pass to CGO or trick to satisfy #ifdef __cplusplus condition?
Compilation exception:
C:\Temp\go-build318595762\cgo_issue\_obj\lib.o: In function `callC_inline':
./lib.h:11: multiple definition of `callC_inline'
C:\Temp\go-build318595762\cgo_issue\_obj\main.cgo2.o:D:/work/go/cgo_issue/lib.h:11: first defined here
collect2.exe: error: ld returned 1 exit status
main.go:
package main
//#cgo CFLAGS: -std=gnu99
//#include "lib.h"
import "C"
import "fmt"
func main() {
fmt.Printf("Go call\n")
C.callC()
}
lib.h:
void callC();
#ifdef __cplusplus
#define TEST_INLINE inline
#else
#define TEST_INLINE
#endif
TEST_INLINE int callC_inline (){
return 1;
}
lib.c:
#include "lib.h"
#include <stdio.h>
void callC(){
printf("C call\n");
}
It is possible to pass some additional info to cgo via special comments. In this case // #cgo CFLAGS: -D__cplusplus
More info here
Related
I just begin new project in eclipse. Try to use C-library from C++ file.
In headers, where are prototypes of functions, here is:
#ifdef __cplusplus
extern "C" {
#endif
// prototypes...
void Init_Configuration(void);
#ifdef __cplusplus
}
#endif
It's initialize.h. In main.cpp i include this file and try use function:
#include "initialize.h"
int main()
{
Init_Configuration();
// Life cycle
while (1)
{
}
}
After compilation i get this error:
initialize.c:54: undefined reference to `SPI_Cmd'
In source C file "initialize.c", where i also include "initialize.h" i use function from C-library for stm32f2xx:
#include "initialize.h"
#include "stm32f2xx_rcc.h"
#include "stm32f2xx_spi.h"
#include "stm32f2xx_gpio.h"
// some code
SPI_Cmd(SPI1, DISABLE);
// some code
In this library also is this code:
#ifdef __cplusplus
extern "C" {
#endif
...
I don't know where a problem. Please, help!
I know there are several questions about this error message but I did not find a suitable solution for my case. I want to export the Filter class from a library. Visual Studio 2013 just compiles fine, but gcc throws the error:
prog.cpp:16:17: error: expected initializer before 'Filter'
class DllExport Filter{
^
prog.cpp:22:6: error: 'Filter' has not been declared
void Filter::setFilter(const std::vector<float>& vFilter, unsigned int uNumThreads) {
^
The code:
#ifndef _GNULINUX
#define DllExport __declspec(dllexport)
#else
#define DllExport __attribute__((visibility("default")))
#endif
#include <string>
#include <vector>
#include <iostream>
#ifdef __cplusplus
extern "C" {
namespace FilterAPI {
#endif
class DllExport Filter{
public:
static void setFilter(const std::vector<float>& vFilter, unsigned int uNumThreads);
};
void Filter::setFilter(const std::vector<float>& vFilter, unsigned int uNumThreads) {
}
#ifdef __cplusplus
} // namespace FilterAPI
} // extern "C" {
#endif
See also
https://ideone.com/3VV4AH
Edit:
flags in the make file are:
# CMAKE generated file: DO NOT EDIT!
# Generated by "Unix Makefiles" Generator, CMake Version 2.8
# compile CXX with /usr/bin/c++
CXX_FLAGS = -fPIC
CXX_DEFINES = -DFilter_Library_EXPORTS
I think that your problem in wrong define,
should be #ifndef __linux, you can find out gcc defaults
with gcc -dM -E - < /dev/null
I don't think you need to explicitly declare the visibility to be "default". It matters on Windows, yes, but on Linux/POSIX the default is what you want.
#ifdef _WIN32
#define DllExport __declspec(dllexport)
#else
#define DllExport
#endif
This is the more common idiom, at least.
In my Eclipse C/C++ project there is this undefined reference error which doesn't go away, doesn't matter what I do. I already changed the link order, checked if all files are compiled and included, basically everything, which is recommended in the in the Internet, when facing this problem.
Here's my program:
[...] - means that there is more code, which doesn't relate.
menulib.h
[...]
void start_GUI( void );
[...]
start_GUI.c - Note this is a .c-File
#include "menulib.h"
void start_GUI( void )
{
[...]
}
coreInterface.h
#ifndef COREINTERFACE_H_
#define COREINTERFACE_H_
#include <stdint.h>
#include <stdlib.h>
#ifdef __cplusplus
/** only include the Core when compiled for C++ */
#include "Core.h"
#endif /* #ifdef CORE */
#ifdef __cplusplus
extern "C" {
#endif
void init_GUI( Core* core);
#ifdef __cplusplus
}
#endif
#endif /* COREINTERFACE_H_ */
coreInterface.cpp - Note this is a .cpp-File
#include "coreInterface.h"
#include "menulib.h"
void init_GUI( Core* core)
{
gui_core = core;
start_GUI(); // <--- **error appears here** - calls a .c file
}
Here's the error in the makefile (the auto generated one from Eclipse):
g++ -L/home/PC/the_Project/menu_GUI -o "the_Project" [...] ./menu_GUI/coreInterface.o [...] ./menu_GUI/start_GUI.o [...] -lcurses
./menu_GUI/coreInterface.o: In function `init_GUI':
/home/PC/the_Project/Default/../menu_GUI/coreInterface.cpp:23: undefined reference to `start_GUI()'
I believe this is coming from merging the .c and .cpp files. Still I couldn't solve it. Has anyone an idea?
Kind Regards
The problem is that you share the menulib.h header between C and C++ without specifying to C++ compiler that the function start_GUI is defined in C. Use extern "C" in #ifdef not only for init_GUI, but also for start_GUI.
Working example:
#include "coreInterface.h"
extern "C" {
#include "menulib.h"
}
void init_GUI( Core* core)
{
gui_core = core;
start_GUI();
}
I tried to link a static library (compiled with gcc) to a c++ program and I got 'undefined reference'. I used gcc and g++ version 4.6.3 on a ubuntu 12.04 server machine. For example, here is the simple library file for factorial method:
mylib.h
#ifndef __MYLIB_H_
#define __MYLIB_H_
int factorial(int n);
#endif
mylib.c
#include "mylib.h"
int factorial(int n)
{
return ((n>=1)?(n*factorial(n-1)):1);
}
I created object for this mylib.c using gcc:
gcc -o mylib.o -c mylib.c
Again the static library was created from the object file using AR utility:
ar -cvq libfact.a mylib.o
I tested this library with a C program (test.c) and C++ program (test.cpp)
Both C and C++ program have the same body:
#include "mylib.h"
int main()
{
int fact = factorial(5);
return 0;
}
Assuming static library libfact.a is available in /home/test directory, I compiled my C program without any issues:
gcc test.c -L/home/test -lfact
However while testing C++ program, it threw a link error:
g++ test.cpp -L/home/test -lfact
test.cpp:(.text+0x2f): undefined reference to `factorial(int)'
collect2: ld returned 1 exit status
I even tried adding extern command in test.cpp:
extern int factorial(int n) //added just before the main () function
Still the same error.
Can someone tell me what I am wrong here?
Is there anything I missed while creating the static library?
Do I have to add anything in my test.cpp to make it work?
The problem is that you haven't told your C++ program that factorial is written in C. You need to change your test.h header file. Like this
#ifndef __MYLIB_H_
#define __MYLIB_H_
#ifdef __cplusplus
extern "C" {
#endif
int factorial(int n);
#ifdef __cplusplus
}
#endif
#endif
Now your header file should work for both C and C++ programs. See here for details.
BTW names containing a double underscore are reserved for the compliler (so are names starting with an underscore and a capital letter) so #ifndef __MYLIB_H_ is illegal strictly speaking. I would change to #ifndef MYLIB_H #define MYLIB_H
While the accepted answer is absolutely correct, I thought I'd just add an observation. Some editors have trouble with the open / close brace, and will indent the entire extern "C" scope in the header. If mylib.h is a key header for a library, you might consider:
#if defined (__cplusplus)
#define _MYLIB_INIT_DECL extern "C" {
#define _MYLIB_FINI_DECL }
#else
#define _MYLIB_INIT_DECL
#define _MYLIB_FINI_DECL
#endif
All other headers in mylib library, e.g., mylib_aux.h, can be of the form:
#ifndef _MYLIB_AUX_H
#define _MYLIB_AUX_H
#include <mylib.h>
_MYLIB_INIT_DECL
... header content ...
_MYLIB_FINI_DECL
#endif /* _MYLIB_AUX_H */
Obviously, the names I'm using are arbitrary, but for multiple library headers, this approach has been useful to me.
I'm compiling a Shared library using Android NDK r6b. All classes are C++.
I have the following two classes:
Utils.hpp
#ifdef USE_OPENGL_ES_1_1
#include <GLES/gl.h>
#include <GLES/glext.h>
#else
#include <GLES2/gl2.h>
#include <GLES2/gl2ext.h>
#endif
#include <android/log.h>
// Utility for logging:
#define LOG_TAG "ROTATEACCEL"
#define LOG(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)
#ifdef __cplusplus
extern "C" {
#endif
static void checkGlError(const char* op);
#ifdef __cplusplus
}
#endif
Utils.cpp
#include "Utils.hpp"
#ifdef __cplusplus
extern "C" {
#endif
static void checkGlError(const char* op) {
for (GLint error = glGetError(); error; error
= glGetError()) {
LOGI("after %s() glError (0x%x)\n", op, error);
}
}
#ifdef __cplusplus
}
#endif
When I want to use this function in other C++ files I #include "Utils.hpp". But, in those files I get an error:
undefined reference to `checkGlError'
Why am I getting this warning?
You've made it static. It only lives in that specific translation unit therefore. The solution is to remove the static keyword.
The warning is telling you that in the header file you "promised" there would be a definition in that translation unity if one was needed, but one has not been provided and it was needed.
static void checkGlError(const char* op);
It is a static function, that means, it has internal linkage, and therefore cannot be called from another translation unit.
Remove the static keyword from it's declaration as well as from it's definition, and it would work fine.