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POAre #include directives processed prior to macro expansion regardless of their location within a file?
 

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  1. POAre #include directives processed prior to macro expansion regardless of their location within a file?
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    <p>I came across some code the other day that was similar to the following (the following has been over-simplified for the sake of brevity):</p> <p><strong>config.h</strong></p> <pre><code>#ifndef __CONFIG__ #define __CONFIG__ #define DEVELOPMENT_BLD _TRUE_ #if (DEVELOPMENT_BLD == _TRUE_) #define FILE_EXT ".dev" #else #define FILE_EXT ".bin" #endif #define PROJECT_STRING "my_project" #define FILE_NAME PROJECT_STRING FILE_EXT /* Common include files */ #include "my_defs.h" #endif /* __CONFIG__ */ </code></pre> <p><strong>my_defs.h</strong></p> <pre><code>#ifndef __MY_DEFS__ #define __MY_DEFS__ #define _TRUE_ 1 #endif /* __MY_DEFS__ */ </code></pre> <p>The project had always compiled without any issues, but since I made some minor changes (and the actual project was rather large) I decided to run Lint on it. When I did, I received the following error:</p> <p><code>Warning 553: Undefined preprocessor variable '_TRUE_', assumed 0</code></p> <p>I then wondered why the compiler didn't catch that <code>_TRUE_</code> is defined in my_defs.h which is included <strong>after</strong> the macro's first usage. So I compiled it on a different compiler with the same results - succesful compilation, no warnings and <code>FILE_NAME</code> was correctly evaluated regardless of how I set <code>DEVELOPMENT_BLD</code> (using <code>_TRUE_</code> or <code>!_TRUE_</code>). Here are my two compiler settings:</p> <p><code>ArmCC -c -cpu Cortex-M3 -g -O0 --apcs=interwork -I "..\ARM\CMSIS\Include" -I "..\ARM\INC\NXP\LPC17xx" -o "file.o" --omf_browse "file.crf" --depend "file.d" "file.c"</code></p> <p><code>mingw32-gcc.exe -pedantic -Wall -g -c D:\dev\practice\header_question\main.c -o obj\Debug\main.o</code></p> <p>I decided to run a simple test to see if the value of <code>FILE_NAME</code> was being properly evaluated by the preprocessor. I also wanted to see what the value of <code>DEVELOPMENT_BLD</code> actually was. I ran the following code two times:</p> <p><strong>main.c</strong></p> <pre><code>#include "config.h" #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; int main() { printf("FILE_NAME:%s, WHAT_IS_TRUE:%d", FILE_NAME,DEVELOPMENT_BLD); return 0; } </code></pre> <p>The first time I used the value <code>#define DEVELOPMENT_BLD _TRUE_</code> with this result:</p> <p><code>FILE_NAME:my_project.dev, WHAT_IS_TRUE:1</code></p> <p>The second time I used the value <code>#define DEVELOPMENT_BLD !_TRUE_</code> with this result:</p> <p><code>FILE_NAME:my_project.bin, WHAT_IS_TRUE:0</code></p> <p>My first thought was that perhaps <code>_TRUE_</code> was being defined elsewhere - so just to be sure I commented out <code>#include "my_defs.h"</code>. I then began to receive a compiler error:</p> <p><code>error: '_TRUE_' undeclared (first use in this function)</code></p> <p>All of that leads to my question. Are #include statements required to be evaluated by the preprocessor before macro expansion or did I just get lucky?</p>
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