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copied!<h1>How to find it out</h1> <p>That's the address of a label and it's a feature specific to <a href="http://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html">GCC</a>.</p> <pre><code>int main(void) { void* startp; s: startp = &&s; printf("the assignment above starts at address %p\n", startp); return 0; } </code></pre> <p>You could have figured it out yourself by testing:</p> <pre><code>int main(void) { void* startp; int a; startp = &&a; printf("startp=%p\n", startp); return 0; } </code></pre> <p>In which case GCC says:</p> <blockquote> <p>error: label ‘a’ used but not defined</p> </blockquote> <h1>Under the hood - assembly</h1> <p>You need to know assembler to really understand this, but I'll try to explain you what an address of a label means.</p> <p>After the OS loads the .exe file from the disk, a component of the operating system called "the loader" (windows has the "PE Loader", linux has "ELF loader" or maybe even others, if they're compiled in the kernel), it does a "virtualization" of that program, turning it into a process.</p> <p>This process thinks it is the only one in RAM and it has access to the entire RAM (that is, 0x00000000-0xFFFFFFFF on a 32-bit machine).</p> <p>(the above is just a short overwiew of what's happenning, you really need to learn assembly to understand it fully, so bear with me)</p> <p>Now, the label in a source code is basically an address. "goto label;" does nothing else than a jump to that address (think about the <a href="http://en.wikipedia.org/wiki/Program_counter">instruction pointer</a> in assembly). This label stores this RAM address, and that's how you can find out that address.</p> <p>After you've learned ASM, you'll realize that that address points to a instruction within the <code>.text</code> section of the executable. The <code>.text</code> section is the one which holds you program's (binary) code to be executed.</p> <p>You can inspect this with:</p> <blockquote> <p>objdump -x a.out</p> </blockquote> <h1>A practical example</h1> <p>As described in <a href="http://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html">GCC</a>, you can use this to initialize a jump table. Some <a href="http://en.wikipedia.org/wiki/Lexical_analysis">scanner generators</a> like <a href="http://re2c.org/">re2c</a> (see the <code>-g</code> parameter) use that to generate more compact scanners. Maybe there's even a <a href="http://en.wikipedia.org/wiki/Compiler-compiler">parser generator</a> employing the same technique.</p>

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