How to compile and link header file in C++?

Question

I am trying to work with the gpio_lib of cubietruck. Therefore I got the gpio_lib.h (see https://github.com/youyudehexie/node-cubieboard-gpio/blob/master/lib/gpio_lib.h) file located in my working directory:

> l
gpio_lib.h
test.cpp

In my test.cpp I have the following include:

#include "gpio_lib.h"

and some calls of the gpio_lib functions like sunxi_gpio_init and sunxi_gpio_output within the main() function.

However if I try to compile and link the send.cpp via

> g++ -o test -O3 -Wall test.cpp

I get

/tmp/ccULAi2f.o: In function `sequence_up()':
test.cpp:(.text+0x26a): undefined reference to `sunxi_gpio_output(unsigned int, unsigned int)'
/tmp/ccULAi2f.o:test.cpp:(.text.startup+0xa2): more undefined references to `sunxi_gpio_output(unsigned int, unsigned int)' follow
collect2: error: ld returned 1 exit status

I also tried following calls getting the same error:

> g++ -o test -O3 -Wall gpio_lib.h test.cpp
> g++ -o test -O3 -Wall -I. -L. gpio_lib.h test.cpp

What is the correct way to compile and link the test.cpp?

That's the answer: I need the implemention too. Thank you! :) — phortx, Oct 04 '14 at 16:12

score 4 · Accepted Answer · answered Oct 04 '14 at 16:06

4

Compile and link the corresponding source file

g++ -o test test.cpp gpio_lib.c

Since the header is pure C, you'll need to add the language linkage yourself if you include it from C++:

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

answered Oct 04 '14 at 16:06

Mike Seymour

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@OP; This is the only correct answer at the time of this posting. – Brandon Oct 04 '14 at 16:17
Right, that's the correct answer. Thank you. – phortx Oct 04 '14 at 16:18

score 2 · Answer 2 · answered Oct 04 '14 at 16:05

2

You cannot link with a header file. The error you are getting is linking error, i.e. function declaration was available in .h file, so it could compile, but definition of function is missing. You need to locate the correct library and link with that

answered Oct 04 '14 at 16:05

Dr. Debasish Jana

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4pie0 · Answer 3 · 2014-10-04T16:34:06.370

By inclusion of header #include "gpio_lib.h" you have brought probably some declarations of functions. Since the header is in C you should add a linkage specifier before #include definition

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

to provide appropriate linkage. Then you have to link to the binaries that are the definitions (the body) of these functions as well or ask the gcc (g++ is gcc module) to do all the things for you:

g++ -o test test.cpp gpio_lib.c

How to compile and link header file in C++?

3 Answers3