Linker error not related to Linker

Question

I have a very specific case of Linker error :

'Invoking: GCC C++ Linker 4 [arm-linux-gnueabihf]' arm-linux-gnueabihf-g++ -lpthread ./src/FPGA_Peripherals/AUX_IMU/AUX_IMU_functions.o
./src/main.o: In function main': ../src/main.cpp:7: undefined reference to `function()'

the error output is truncated for purpose of this post. The error and the object, where the function definitions lies, is highlighted.

The code is compiled and linked using DS-5 C/C++ Eclipse Platform, with the GCC 4.x [arm-linux-gnueabihd] (DS-5 built-in) tool-chain :

GCC C++ Compiler 4 [arm-linux-gnueabihf]
GCC C Compiler 4 [arm-linux-gnueabihf]
GCC Assembler 4 [arm-linux-gnueabihf]
GCC C Linker 4 [arm-linux-gnueabihf]
GCC C++ Linker 4 [arm-linux-gnueabihf]
GCC Archiver 4 [arm-linux-gnueabihf]

With Gnu Make Builder.

Source code is structured in folders :

src

main.cpp
- FPGA_peripherals
  - AUX_IMU
  header.h
  
  AUX_IMU_functions.c

Minimalistic code producing the error :

main.cpp

#include "header.h"

int main() {
    function();
    return 0;
}

header.h

void function(void);

AUX_IMU_functions.c

#include "header.h" 

void function(void){
    int i = 3;
};

The C code is correctly compiled using GCC C Compiler 4 [arm-linux-gnueabihf]. The C++ code (other files, not included within this example), are correctly compiled using GCC C++ Linker 4 [arm-linux-gnueabihf].

This apparently is not a Linker related problem, but what else to check, if Linker still produces this error ?

The error disappears, once I re-name the files to .hpp and .cpp. Why is that ? Does GCC C and GCC C++ produces incompatible .o objects ?

Note that the final semicolon in `void function(void){ int i = 3; };` is superfluous. Almost, but not quite harmless; under strict compiler options, you'll get a warning about an empty global declaration, which will probably confuse you. Initializers and types need a semicolon after the close brace; functions do not. — Jonathan Leffler, Jan 13 '18 at 10:47
Your error occurs because of C++ type-safe linkage which mangles function names. You need to tell the C++ compiler that the `function()` has C linkage: `extern "C" void function(void);`. You normally do that by using `#ifdef __cplusplus` / `extern "C"` / `#endif` before the function declaration, or use the `extern "C" {` before and `}` conditionally after a block of C function declarations. — Jonathan Leffler, Jan 13 '18 at 10:48
@JonathanLeffler well, it is a constraint violation, so the compilation is allowed to fail. — Antti Haapala -- Слава Україні, Jan 13 '18 at 10:57

score 0 · Accepted Answer · answered Jan 13 '18 at 10:54

Your error occurs because of C++ type-safe linkage, which mangles function names. You need to tell the C++ compiler that the function() has C linkage:

extern "C" void function(void);

However, if the same header should be used by both the C and C++ compilers, you normally do that by using

#ifdef __cplusplus
extern "C"
#endif
void function(void);

for a single function declaration, or use

#ifdef __cplusplus
extern "C" {
#endif 

void function(void);
int  response(int arg);
…

#ifdef __cplusplus
}
#endif

around a block of function declarations for functions with C linkage.

You can also use your existing header in C, and in your C++ code, use:

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

Thank you. I have actually tried to use 'extern "C" ' before, but it produces other error, because of the GCC C compiler, which doesnt recognizes it. Now I understand why it is so vital to add the #ifdef _cplusplus — new_stacker, Jan 13 '18 at 11:24

Linker error not related to Linker

1 Answers1