General :: Return -1 Of Main But Get Exit Code Is 255? Why? Pls Advise.

I learn C++ reading the <C++ primer 4th>, question1.2 let me test if return -1 how the compiler handle this exit code.
and in gcc (GCC) 4.1.1 20070105 (Red Hat 4.1.1-52), I get this show:

[youxi600@Arthas C++ Primer]$ cat q1_2.cpp
int main()
{
return -1;
}
[youxi600@Arthas C++ Primer]$ gcc q1_2.cpp -o q1_2
[youxi600@Arthas C++ Primer]$ ./q1_2
[youxi600@Arthas C++ Primer]$ echo $?
255
[youxi600@Arthas C++ Primer]$
anyone can give me an advise? Many thanks!