Main is usually a function. So then when is it not? (2015)

Well, if you really want to confound things you could write your own __start which is typically the entry point called by the kernel, which in turn calls `main()`.<p>But actually that&#x27;s just by convention: C (and C++) programs drop `__start` into the binary and then tell the linker to mark the binary (mostly an ELF file these days, thank goodness) to indicate that the entry point is the symbol `__start`. There can be different versions depending on how many arguments to `main()` are requested, or it can simply pass all three and let the function body ignore the unwanted ones.<p>If you <i>really</i> want to confuse the TA, write the following program:<p><pre><code> #include &lt;stdio.h&gt; void foo(); int main() { printf (&quot;I&#x27;m in main\n&quot;); foo(); printf (&quot;I&#x27;m still in main\n&quot;); } void foo() { printf (&quot;I&#x27;m in foo!\n&quot;); } </code></pre> Then <i>tell the linker to mark `foo` as your entry point</i>. You could do this from the command line or even a custom linker script.<p>Of course depending on what your OS needs from `__start` you may have to do some extra setup, sorry. But this might work on Linux.<p>I&#x27;ve actually been thinking of modifying gcc&#x27;s `__start` to pass the arguments and env vars as a span (well, the function __start calls that calls `main()`.)

This is fun. On a broader note though, entry point main-type functions have always bothered me. Maybe because I grew up as a simple script kiddie with BASIC and Bash and PHP. I like code that starts executing from the first line and then runs whatever functions it wants to run. I realize that&#x27;s just an abstraction of main() but it&#x27;s a pleasant one for me. There&#x27;s something more constricting about there being one function to bootstrap everything than there is about one file.

Complete final program for the laziest of us (after incorporating @10000truths&#x27;s advice):<p><pre><code> const int main[] __attribute__ ((section(&quot;.text&quot;))) = { -443987883, 440, 113408, -1922629632, 4149, 899584, 84869120, 15544, 266023168, 1818576901, 1461743468, 1684828783, -1017312735 }; </code></pre> Compilation (gcc 13):<p><pre><code> $ gcc -Wall main.c -o main main.c:1:11: warning: ‘main’ is usually a function [-Wmain] 1 | const int main[] __attribute__ ((section(&quot;.text&quot;))) = { | ^~~~ &#x2F;tmp&#x2F;ccsWmdiD.s: Assembler messages: &#x2F;tmp&#x2F;ccsWmdiD.s:4: Warning: ignoring changed section attributes for .text </code></pre> Execution:<p><pre><code> $ .&#x2F;main Hello World!</code></pre>

This won&#x27;t work anymore, as compilers will now place const arrays in the .rodata section, which is non-executable. Luckily, there&#x27;s an easy fix - just qualify the declaration of the array with:<p>__attribute__((section(&quot;.text&quot;))

Back when compiler warnings possibly cost extra processing time to generate, it was possible to make gcc compile the craziest things.<p>After much experimentation, it turned out that the smallest program that would compile and run was only 5 bytes long:<p>main;

We built a embedded &quot;Operating System&quot; during CS science courses, that was basically just a continous recursion, were the stack was eliminated and reset repeatetly. Main was just some assembly manipulating the instruction pointer to get the whole thing rolling. Good times, good crimes.

This reminds me of James Iry saying &quot;all your code and data are in one giant mutable array indexed by pointers, good luck&quot;. Half true, but insightful.

(2015)

Looks like he&#x27;s independently re-invented PEEK and POKE from BASIC.

5 points off for coding style.<p>5 points off for assuming too much about the system.

The article says `lea` helps calculate the array relative address on AMD64.<p>Why does the article say the problem would be tricky on 32-bit? `lea` is an old instruction. Thanks.

This reminds me of a great riddle: what is the shortest C code that compiles, but segfaults?<p>With the right flags: `main;` (implicitly a zeroed int).

main;<p>is a valid C program but main is not a function here.