CODEWITHSUNDEEP
c++clinuxundefined-reference
Rayne
Rayne

Reputation: 14977

Undefined Reference to a function

I'm using Linux and I have the following files:

main.c, main.h
fileA.c, fileA.h
fileB.cpp, fileB.h

The function F1() is declared in fileB.h and defined in fileB.cpp. I need to use the function in fileA.c, and so I declared the function as 

extern void F1();

in fileA.c.

However, during compilation, I got the error 

fileA.c: (.text+0x2b7): undefined reference to `F1'

What is wrong?

Thank you.

ETA: Thanks to the answers I've received, I now have the following:

In fileA.h, I have

#include fileB.h
#include main.h

#ifdef __cplusplus
extern "C" 
#endif
void F1();

In fileA.c, I have

#include fileA.h

In fileB.h, I have

extern "C" void F1();

In fileB.cpp, I have

#include "fileB.h"

extern "C" void F1()
{ }

However, I now have the error

fileB.h: error: expected identifier or '(' before string constant

Upvotes: 4

Views: 18932

Answers (5)

dmh2000
dmh2000

Reputation: 683

fileA.c can't include fileB.h (via fileA.h) because the C compiler doesn't know what extern "C" means, so it complains that it sees an identifier before a string. don't try to include fileB.h in fileA.c or fileA.h. its not needed

Upvotes: 2

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

If you're really compiling fileA.c as C, not C++, then you need to make sure that the function has the proper, C-compatible linkage.

You can do this with a special case of the extern keyword. Both at declaration and definition:

extern "C" void F1();
extern "C" void F1() {}

Otherwise the C linker will be looking for a function that only really exists with some mangled C++ name, and an unsupported calling convention. :)

Unfortunately, whilst this is what you have to do in C++, the syntax isn't valid in C. You must make the extern visible only to the C++ code.

So, with some preprocessor magic:

#ifdef __cplusplus
extern "C"
#endif
void F1();

Not entirely pretty, but it's the price you pay for sharing a header between code of two languages.

Upvotes: 19

Alok Save
Alok Save

Reputation: 206526

To be able to call a c++ function from c source code you neeed to give the appropriate linkage specification.

The format for specifying linkage specification is 

extern "type_of_Linkage" <function_name>

So in your case, you should be using:

extern "C" void F1();

Upvotes: 5

mattn
mattn

Reputation: 7723

perhaps, use

extern "C" void F1();

Upvotes: 4

Xedecimal
Xedecimal

Reputation: 3223

fileA.c needs to also include fileA.h I believe.

Upvotes: -1

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