What's the "unix" variable is?

[lilydjwg]

I found that there is a variable called "unix" which I don't know who has defined. Its size is 4 bytes and its value is integer 1.
My OS is Linux, and the compiler is g++ 4.3.3.
Who can explain where the variable comes from? Or who has some idea of the variable?

[helios]

test.cpp: In function `int main()':
test.cpp:4: error: `unix' undeclared (first use this function)
test.cpp:4: error: (Each undeclared identifier is reported only once for each function it appears in.)

Must be defined in your code.

[Duthomhas]

It isn't a variable -- it is a macro, defined to identify that your OS is a *nix environment.
http://predef.sourceforge.net/index.php

[lilydjwg]

OK, I see that. No wonder the compiler told me unix can't be lvalue!
Thanks!