main之前和之后（可）执行哪些函数（操作）

#include <stdlib.h>
#include <stdio.h>

/* Prototypes */
int fn1(void), fn2(void), fn3(void), fn4 (void);

void main( void )
{
_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;
}

Output
This is executed first.
This is executed next.