function called from .cpp file which is defined in in .c file

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.

@dasblinkenlight: No, the error message for that is "error: ‘functionA’ was not declared in this scope" — Dietrich Epp
– Dietrich Epp, Commented Jul 13, 2012 at 17:18
Yes, I have included it. Sorry I forgot to mention it here. Thanks. I made an edit just now. — RRR
– RRR, Commented Jul 13, 2012 at 17:19
You should show the command line that is producing the error. — Jonathan Leffler
– Jonathan Leffler, Commented Jul 13, 2012 at 17:21

Dietrich Epp · Accepted Answer · 2012-07-13 17:23:30Z

4

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.

$ ls
fileA.c  fileA.h  fileB.cpp  fileB.h
$ gcc -c fileA.c
$ g++ -c fileB.cpp
fileA.c  fileA.h  fileA.o  fileB.cpp  fileB.h  fileB.o
$ g++ fileA.o fileB.o
$ ls
a.out  fileA.c  fileA.h  fileA.o  fileB.cpp  fileB.h  fileB.o

answered Jul 13, 2012 at 17:23

Dietrich Epp

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Comments

John Stamos · Accepted Answer · 2012-07-13 17:16:29Z

1

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

answered Jul 13, 2012 at 17:16

John Stamos

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Comments

Jonathan Leffler · Accepted Answer · 2012-07-13 17:23:45Z

1

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();
}

edited Jul 13, 2012 at 17:23

answered Jul 13, 2012 at 17:15

Jonathan Leffler

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1 Comment

Sergey Kalinichenko Over a year ago

+1 Linking issue is certainly the problem, because the error message points to the linker.

Jonathan Leffler · Accepted Answer · 2012-07-13 17:17:28Z

0

How do you compile?

gcc filea.c fileb.cpp

Should do the Trick.

edited Jul 13, 2012 at 17:17

Jonathan Leffler

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answered Jul 13, 2012 at 17:16

Matthias

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2 Comments

Jonathan Leffler Over a year ago

That probably won't work; gcc probably will reject the C++ source, and it doesn't address the source code level issues in the C++ code.

Dietrich Epp Over a year ago

@JonathanLeffler: It will compile the C++ source as C++ (GCC chooses language based on extension by default) but it won't link in the standard C++ library, which will probably cause link errors.

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