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From: Qun-Ying <zhu.qunying@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: GPS: Including C headers from paths which are not configured in
 Source_Dirs
Date: Thu, 8 Oct 2015 11:11:39 -0700
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Xref: news.eternal-september.org comp.lang.ada:27941
Date: 2015-10-08T11:11:39-07:00
List-Id: <comp.lang.ada>

Rego, P. wrote:
> On Thursday, October 8, 2015 at 12:29:32 PM UTC-3, Simon Wright wrote:
>>     package Compiler is
>>        for Switches ("C") use ("-I/your/include/dir");
>>     end Compiler;
>
> It did not work, maybe I did wrong. I built a test code to analyze better.
>
> --test.gpr
> project Test is
>     for Languages use ("Ada", "C");
>     for Source_Dirs use (".");
>     for Main use ("ohmy.adb");
> 	
>     package Compiler is
>        for Switches ("C") use ("-Ioutsiders");
>     end Compiler;
>
> end Test;
>
> --ohmy.adb
> procedure Ohmy is
>     procedure Put_Ohmy;
>     pragma Import (C, Put_Ohmy, "putOhmy");
> begin
>     Put_Ohmy;
> end Ohmy;
>
> --bla.h
> void putOhmy(void);
>
> --bla.c
> #include <bla.h>
> #include "common/bla_other.h"
>
> void putOhmy (void) {
>      putOhmyOther();
> }
>
> --outsiders/common/bla_other.h
> void putOhmyOther();
>
> --outsiders/common/bla_other.c
> #include <bla_other.h>
> #include <stdio.h>
>
> void putOhmyOther(){
>      printf("Ohmy");
> }
>
> In this case, the built returns the error message:
>> gprbuild -d -Ptest.gpr ohmy.adb
>> gcc ohmy.o -o ohmy.exe
>> libtest.a(bla.o):bla.c:(.text+0x7): undefined reference to `putOhmyOther'
>> collect2.exe: error: ld returned 1 exit status
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Your program compile fine, it include your header file without problem, 
but failed on linking.

As putOhmyOther is defined in common/bla_other.c, which is not compiled 
in your project.

>> gprbuild: link of ohmy.adb failed
>> [2015-10-08 13:38:54] process exited with status 4, 100% (2/2), elapsed time: 03.94s
>
> Realize that the folder 'outsiders' is included in project, so (I guess)
 > its subfolders should be accessible by an include like
 > '#include "common/bla_other.h"' (?) So what did I do wrong?

If you are linking with 3rd party C lib, not only do you need to include 
the header file, you also need to link with its lib.