exit库函数和_exit系统调用对标准输出的影响

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>

int globvar = 6;
char buf[] = "a write to stdout\n";

int main(void)
{
    int var;
    pid_t pid;
    
    var = 88;
    if(write(STDOUT_FILENO, buf, sizeof(buf)-1) != sizeof(buf)-1)    //write系统调用输出，没有缓冲区
    {
        exit(1);
    }
    printf("before fork!\n");        //标准库函数输出，内容会存到缓冲区内
    
    if((pid = fork()) < 0) {
        perror("fork error!");
        exit(1);
    }else if(pid == 0) {
        globvar++;
        var++;
    }else {
        sleep(2);
    }
    
    printf("pid = %ld, glob = %d, var = %d\n", (long)getpid(), globvar, var);
    
    _exit(0);        //采用_exit系统调用不会flush缓冲区的内容到文件
    //exit(0);        //采用exit库函数在退出之前会将缓冲区的内容flush到文件
}