从.cpp文件中调用的函数，该文件在.c文件中定义

Question

In projectA.vcproj

fileA.h

#ifdef __cplusplus
extern "C" {
#endif
void functionA();
#ifdef __cplusplus
}
#endif

fileA.c

void functionA()
{
//function defined
}

In projectB.vcproj:

fileB.h

void functionB() ;

fileB.cpp

#include "fileA.h"
#include "fileB.h"
void functionB() {
    functionA(); // error: undefined reference to 'functionA'
}

I am getting the error when I compile my code on Linux, please help me fix this.

Answer 1

You have to link the files together.

Source code ---compile--> Object files ---link--> Application

fileA.c ------------+
                    |---> fileA.o ---------+
                +---+                      |
                |                          |
fileA.h --------+                          +--> a.out
                |                          |
                +---+                      |
                    |---> fileB.o ---------+
fileB.cpp ----------+

The "undefined reference to XXX" error message is given by the linker, after successful compilation.

You need to make sure all files are linked together.

$ 
fileA.c  fileA.h  fileB.cpp  fileB.h
$ 
$ 
fileA.c  fileA.h  fileA.o  fileB.cpp  fileB.h  fileB.o
$ 
$ 
a.out  fileA.c  fileA.h  fileA.o  fileB.cpp  fileB.h  fileB.o

Answer 2

The error message is probably coming from the linker, so you need to ensure you compile both source files and link them properly:

gcc -c fileA.c
g++ -c fileB.cpp
g++ -o program fileB.o fileA.o

You should, of course, ensure that fileA.c includes fileA.h . If you omit the header from fileA.c and if you compile the code using:

g++ -c fileA.c                       # g++ instead of gcc
g++ -c fileB.cpp
g++ -o program fileB.o fileA.o

Then you will get the missing reference because g++ will have created a C++ linkage functionA() but will be expecting to call a C linkage functionA() .

However, you should not compile C code with g++ ; that is asking for trouble.

---

When originally asked, fileB.cpp didn't include any headers.

fileB.cpp

#include "fileB.h"
#include "fileA.h"  // Provide extern "C" declaration of functionA()
void functionB() {
    functionA();
}

Answer 3

You need to include the header file of functionA in functionB's header file. So in fileB.h add the line #include "fileA.h"

Answer 4

How do you compile?

gcc filea.c fileb.cpp

Should do the Trick.