写（）到标准输出和printf输出不交叉存取？ [英] write() to stdout and printf output not interleaved?

问题描述

#include <stdio.h>
#define MAXLEN 256

int main() {
  int n;
  char buf[MAXLEN];
  while((n = read(0,buf,sizeof(buf))) != 0){
    printf("n: %d:",n);
    write(1,buf,n);
  }
  return 1;
}

的程序的输出是

The output of the program is

read
read
write
write
n: 5:n: 6:

的printf的输出来pressing后ctrl + D的标准输入，而不是非常久远的后续读取。 Ÿ会出现这种情况？

The output of printf comes after pressing Ctrl+D at the standard input and not alongwith the subsequent reads. Y does this happen?

推荐答案

printf的缓冲。

Printf is buffered.

您可以使用fflush调用强制printf的'刷新'它的缓存：

You can force printf to 'flush' its buffer using the fflush call:

#include <stdio.h>
#define MAXLEN 256

int main() {
  int n;
  char buf[MAXLEN];
  while((n = read(0,buf,sizeof(buf))) != 0){
    printf("n: %d:",n);
    fflush(stdout); /* force it to go out */
    write(1,buf,n);
  }
  return 1;
}

在一般情况下，的printf（）被缓冲是一件好事。无缓冲输出，特别是那些需要屏幕更新和可见这样的游戏机，是缓慢的。慢，以至于被printf'ing大量的应用程序可以通过它直接趋缓（尤其是Windows平台，Linux和Unix系统通常影响更小）。

In general, printf() being buffered is a good thing. Unbuffered output, particularly to visible consoles that require screen updates and such, is slow. Slow enough that an application that is printf'ing a lot can be directly slowed down by it (especially on the Windows platform; Linux and unixes are typically impacted less).

然而，的printf（）被缓冲确实咬你，如果你还 fprintf中（标准错误，） - 标准错误是故意无缓冲。因此，你可能会与某些的printf（）缺失让你的消息;如果你写的另一个文件处理，它也与终端有关，可能是无缓冲，确保你第一次明确地 fflush（标准输出）。

However, printf() being buffered does bite you if you also fprintf(stderr,) - stderr is deliberately unbuffered. As a consequence, you may get your messages with some printf() missing; if you write to another FILE handle that is also associated with the terminal, and might be unbuffered, make sure you first explicitly fflush(stdout).

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