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：

 1 #include "apue.h"
 2 
 3 int globvar = 6;
 4 char buf[] = "a write to stdout
";
 5 
 6 int main()
 7 {
 8         int var;
 9         pid_t pid;
10     
11         var = 88; 
12         if (write(STDOUT_FILENO, buf, sizeof(buf)-1) !=  
13                 sizeof(buf) -1) 
14         {   
15                 err_sys("write error"); 
16         }   
17         printf("before fork  ");
18         sleep(3);
19         if ((pid = fork()) < 0)
20                 err_sys("fork error");
21         else if (pid == 0) {
22                 globvar++;
23                 var++;
24         } else {
25                 sleep(2);
26         }   
27     
28         printf("pid = %ld, glob = %d, var = %d
",
29                 (long)getpid(), globvar, var);
30         exit(0);
31 
32 }

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