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
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.
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();
perhaps, use
extern "C" void F1();
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
fileA.c needs to also include fileA.h I believe.
来源：https://stackoverflow.com/questions/6423340/undefined-reference-to-a-function