main函数执行完毕后，是否可能会再执行一段代码？

#include <cstdlib>
int atexit(void (*function)(void));
#include <cstdlib>
#include <cstdio>

void fn1(void), fn2(void), fn3(void), fn4(void);

int main(void)
{
    atexit(fn1);
    atexit(fn2);
    atexit(fn3);
    atexit(fn4);
    printf("This is executed first.\n");
}

void fn1()
{
    printf("next.\n");
}

void fn2()
{
    printf("executed ");
}

void fn3()
{
    printf("is ");
}

void fn4()
{
    printf("This ");
}

#include <cstdlib>
#include <cstdio>

int fn1(void), fn2(void), fn3(void), fn4(void);

int 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;
}