write不带缓冲，标准IO库带缓冲

结论

使用write函数直接输出到输出设备上，是不带缓冲；但是标准IO库是带有缓冲的，printf遇到 的时候才会冲刷缓冲区，输出到输出设备上。

测试程序

如下：

#include "apue.h"

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

int main()
{
        int var;
        pid_t pid;
    
        var = 88; 
        if (write(STDOUT_FILENO, buf, sizeof(buf)-1) !=  
                sizeof(buf) -1) 
        {   
                err_sys("write error"); 
        }   
        printf("before fork  ");
        sleep(3);
        if ((pid = fork()) < 0)
                err_sys("fork error");
        else if (pid == 0) {
                globvar++;
                var++;
        } else {
                sleep(2);
        }   
    
        printf("pid = %ld, glob = %d, var = %d
",
                (long)getpid(), globvar, var);
        exit(0);

}

输出结果

如下

a write to stdout

// sleep 3s
before fork  pid = 27219, glob = 7, var = 89

// sleep 2s
before fork  pid = 26934, glob = 6, var = 88