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POInclusion of unused symbols in object files by compiler in C vs C++
 

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  1. POInclusion of unused symbols in object files by compiler in C vs C++
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    <p>This might be a dumb question, but maybe someone can provide some insight.</p> <p>I have some global variables defined in a header file (yes yes I know that's bad, but this is just a hypothetical situation). I include this header file in two source files, which are then compiled into two object files. The global symbols are not referenced anywhere in the code.</p> <p>If the source files are C, then it looks like the compiler omits the global symbols and everything links without errors. If the source files are C++, the symbols are included in both object files and then I get linker errors. For C++ I am using extern "C" when I include the header.</p> <p>I am using the Microsoft compiler from VS2005.</p> <p>Here is my code:</p> <p>Header file (test.h):</p> <pre><code>#ifndef __TEST_H #define __TEST_H /* declaration in header file */ void *ptr; #endif </code></pre> <p>C Source files:</p> <p>test1.c</p> <pre><code>#include "test.h" int main( ) { return 0; } </code></pre> <p>test2.c</p> <pre><code>#include "test.h" </code></pre> <p>C++ Source Files:</p> <p>test1.cpp</p> <pre><code>extern "C" { #include "test.h" } int main( ) { return 0; } </code></pre> <p>test2.cpp</p> <pre><code>extern "C" { #include "test.h" } </code></pre> <p>For C, the object files look something like this:</p> <pre><code>Dump of file test1.obj File Type: COFF OBJECT COFF SYMBOL TABLE 000 006DC627 ABS notype Static | @comp.id 001 00000001 ABS notype Static | @feat.00 002 00000000 SECT1 notype Static | .drectve Section length 2F, #relocs 0, #linenums 0, checksum 0 004 00000000 SECT2 notype Static | .debug$S Section length 228, #relocs 7, #linenums 0, checksum 0 006 00000004 UNDEF notype External | _ptr 007 00000000 SECT3 notype Static | .text Section length 7, #relocs 0, #linenums 0, checksum 96F779C9 009 00000000 SECT3 notype () External | _main 00A 00000000 SECT4 notype Static | .debug$T Section length 1C, #relocs 0, #linenums 0, checksum 0 String Table Size = 0x0 bytes </code></pre> <p>And for C++ they look something like this:</p> <pre><code>Dump of file test1.obj File Type: COFF OBJECT COFF SYMBOL TABLE 000 006EC627 ABS notype Static | @comp.id 001 00000001 ABS notype Static | @feat.00 002 00000000 SECT1 notype Static | .drectve Section length 2F, #relocs 0, #linenums 0, checksum 0 004 00000000 SECT2 notype Static | .debug$S Section length 228, #relocs 7, #linenums 0, checksum 0 006 00000000 SECT3 notype Static | .bss Section length 4, #relocs 0, #linenums 0, checksum 0 008 00000000 SECT3 notype External | _ptr 009 00000000 SECT4 notype Static | .text Section length 7, #relocs 0, #linenums 0, checksum 96F779C9 00B 00000000 SECT4 notype () External | _main 00C 00000000 SECT5 notype Static | .debug$T Section length 1C, #relocs 0, #linenums 0, checksum 0 String Table Size = 0x0 bytes </code></pre> <p>I notice that _ptr is listed as UNDEF when I compile the C source, and it is defined when I compile the C++ source, which results in linker errors.</p> <p>I understand that this is not a good thing to do in real life, I am just trying to understand why this is different.</p> <p>Thanks.</p>
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    1. COThere isn't really such a thing as a 'global' variable or symbol in C or C++. So, post your code to show what you've actually done.
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    2. CONot just the code. We need to understand how you are building the application. Are you using static dynamic libs or just object files etc.
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    3. COI added the code to the post. I am just trying to link two object files together into an executable.
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