main 主函数执行完毕后，是否可能会再执行一段代码，给出说明？

可以，可以用_onexit 注册一个函数，它会
在main 之后执行int fn1(void), fn2(void), fn3(void),
fn4 (void);
void main( void )
{
String str("zhanglin");
_onexit( fn1 );
_onexit( fn2 );
_onexit( fn3 );
_onexit( fn4 );
printf( "This is executed first.\n" );
}
int fn1()
{
printf( "next.\n" );
return 0;
}
int fn2()
{
printf( "executed " );
return 0;
}
int fn3()
{
printf( "is " );
return 0;
}
int fn4()
{
printf( "This " );
return 0;
}
The _onexit function is passed the address of a
function (func) to be called when the program
terminates normally. Successive calls to _onexit create
a register of functions that are executed in LIFO
(last-in-first-out) order. The functions passed to
_onexit cannot take parameters.