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    <p>This appears to be an (undocumented) "GNU extension": [<em>correction</em>: I finally found a mention in the docs. See below.]</p> <p>The following command uses the <code>-dM</code> option to print all preprocessor defines; since the input "file" is empty, it shows exactly the predefined macros. It was run with gcc-4.7.3 on a standard ubuntu install. You can see that the preprocessor is standard-aware. In total, there 243 macros with <code>-std=gnu99</code> and 240 with <code>-std=c99</code>; I filtered the output for relevance. </p> <pre><code>$ cpp --std=c89 -dM &lt; /dev/null | grep linux #define __linux 1 #define __linux__ 1 #define __gnu_linux__ 1 $ cpp --std=gnu89 -dM &lt; /dev/null | grep linux #define __linux 1 #define __linux__ 1 #define __gnu_linux__ 1 #define linux 1 $ cpp --std=c99 -dM &lt; /dev/null | grep linux #define __linux 1 #define __linux__ 1 #define __gnu_linux__ 1 $ cpp --std=gnu99 -dM &lt; /dev/null | grep linux #define __linux 1 #define __linux__ 1 #define __gnu_linux__ 1 #define linux 1 </code></pre> <p>The "gnu standard" versions also <code>#define unix</code>. (Using <code>c11</code> and <code>gnu11</code> produces the same results.)</p> <p>I suppose they had their reasons, but it seems to me to make the default installation of gcc (which compiles C code with <code>-std=gnu89</code> unless otherwise specified) non-conformant, and -- as in this question -- surprising. Polluting the global namespace with macros whose names don't begin with an underscore is not permitted in a conformant implementation. (6.8.10p2: "Any other predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second underscore," but, as mentioned in Appendix J.5 (portability issues), such names are often predefined.)</p> <p>When I originally wrote this answer, I wasn't able to find any documentation in gcc about this issue, but I did finally discover it, not in <a href="https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/index.html#toc_C-Implementation">C implementation-defined behaviour</a> nor in <a href="https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/C-Extensions.html#C-Extensions">C extensions</a> but in the <code>cpp</code> manual section <a href="https://gcc.gnu.org/onlinedocs/gcc-5.3.0/cpp/System-specific-Predefined-Macros.html">3.7.3</a>, where it notes that:</p> <blockquote> <p>We are slowly phasing out all predefined macros which are outside the reserved namespace. You should never use them in new programs&hellip;</p> </blockquote>
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    1. POWhy does the C preprocessor interpret the word "linux" as the constant "1"?
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    1. COIt is not a violation of the C standard because GCC compiling for gnu89 or gnu99 is not an implementation of the C standard, and the C standard does not impose any requirements on things that are not implementations of it.
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    2. CO@EricPostpischil: That's certainly true; it's simply a non-conforming implementation. However, -std=c89 is the *default* setting for gcc, so by default gcc is non-conforming. That's not illegal, of course, but one might expect that at some point in the progression of extensions gnu89, gnu99, gnu11, that gcc would either drop the namespace pollution or document it somewhere.
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    3. CO@EricPostpischil: OK, I found some documentation in the gnu cpp docs, which include the sentence " We are slowly phasing out all predefined macros which are outside the reserved namespace. You should never use them in new programs, and we encourage you to correct older code to use the parallel macros whenever you find it." I'll edit my answer after lunch with a pointer.
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