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    <p>I tried your to compile your files into shared libs and then use them (I use cygwin).</p> <p>Here's for Foo:</p> <pre><code>cm@Gregor-PC ~/src $ gcc -I. -c --shared -o libFoo.so Foo.c </code></pre> <p>With the bin util nm you can check the symbols (grep for 'foo' to limit the output)</p> <pre><code>cm@Gregor-PC ~/src $ nm libFoo.so | grep foo 0000000a T _foo U _foobar </code></pre> <p>Where it gives an offset and says 'T' for terminated that tells you the symbol is defined in the lib.</p> <p>Now the FooBar lib has to linked against Foo in order to have the foo symbol</p> <pre><code>cm@Gregor-PC ~/src $ gcc -I. -L. --shared -o libFooBar.so FooBar.c libFoo.so cm@Gregor-PC ~/src $ nm libFooBar.so | grep foo 61f0111e T _foo 61f010e0 T _foobar </code></pre> <p>With this I can compile against FooBar only and get foo as a known symbol:</p> <pre><code>cm@Gregor-PC ~/src $ gcc -I. -o tst1 tst1.c libFooBar.so cm@Gregor-PC ~/src $ ./tst1 Function returned: 42 </code></pre> <p>So it seems to work OK.</p> <p>You can improve on your approach to modularize your C code by having header files that contain only public data types, exported function prototypes (declared as extern) and maybe even public global variables or constants. Such a header file declares the interface to the module and would have to be included where the module is used. </p> <p>This is explained with much more detail in the wonderful book <a href="http://rads.stackoverflow.com/amzn/click/0201419505" rel="nofollow">'Functional C'</a> (Hartel, Muller, 1997, Addison Wesley, <a href="http://eprints.eemcs.utwente.nl/1077/02/book.pdf" rel="nofollow">link</a> ) in the chapter on modules.</p> <p>The benefit is that your dependencies are clearer to see (as includes in the source files) and you don't have to have that unsightly extern declaration in the Foo source. </p> <p>For your example:</p> <pre><code>/* Foo.h */ extern int foo(int id); /* exported to FooBar */ /* FooBar.h */ extern int foobar(); /* export to Foo */ extern void do_something_else(); /* export to tst1 */ /* Foo.c */ #include &lt;Foo.h&gt; #include &lt;FooBar.h&gt; int do_something() { return 11; } int foo(int id){ int var; switch (id){ case 1: var = do_something(); break; case 2: /* the module that gets here, has the required functions defined in it */ var = foobar(); } return var; } /* FooBar.c */ #include &lt;stdio.h&gt; #include &lt;Foo.h&gt; #include &lt;FooBar.h&gt; int foobar(){ return 42; } void do_something_else(){ int ret = foo(2); printf("Function returned: %d", ret); } </code></pre>
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    1. PORefactoring legacy C code - using extern declarations to help split up modules (potential linking and run time issues)
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    1. USGregor Ophey
    1. USGregor Ophey
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    1. COThe 'T' in the `nm` output is for 'text', meaning it is the text (body, code) of a function. 'U' is for undefined, and you get symbols such as 'D' for initialized data or 'C' for data that is in the common section.
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      1. USJonathan Leffler
    2. CO@GregoryOphey: If the extern declaration is moved to Foo.h (which is #included in FooBar.h), then FooBar.h will 'see' two different prototypes for the foobar() function (or am I missing something?)
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      1. USHomunculus Reticulli
    3. CO@HomunculusReticulli: In the code I wrote there are no include statements in the header files, which is part of the beauty of this approach. In general you would only include a header A.h in a header file B.h, if B.h needs a data type defined defined in A.h. You could refine the method by having separate header files for data types and extern declarations ...
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      1. USGregor Ophey
 

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