为什么在串行端口编程中使用fcntl(fd，F_SETFL，0) [英] Why fcntl(fd, F_SETFL, 0) use in serial port programming

问题描述

我正在Linux中开始串行端口编程.在阅读了网上的几个示例后，我不了解fcntl(fd, F_SETFL, 0)的确切效果吗?它正在清除位，但是它会影响哪些标志?它设置和/或清除了什么?

I am starting serial port programming in Linux. After reading several examples on the web, I don't understand exact effect of fcntl(fd, F_SETFL, 0)? It is clearing bits, but what flags does it affect? What does it set and or clear?

推荐答案

一个接一个

1)使用了函数调用

fcntl()-它对传入参数中的文件描述符执行操作.

fcntl() - It perform the operation on file descriptor passed in argument.

2)通话中的第二个参数

2) 2nd argument in call

F_SETFL (int)

将文件状态标志设置为arg指定的值.档案存取 模式(O_RDONLY，O_WRONLY，O_RDWR)和文件创建标志(即 arg中的O_CREAT，O_EXCL，O_NOCTTY，O_TRUNC)将被忽略.在Linux上， 命令只能更改O_APPEND，O_ASYNC，O_DIRECT，O_NOATIME， 和O_NONBLOCK标志.

Set the file status flags to the value specified by arg. File access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored. On Linux this command can change only the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.

3)通话中的第三个参数

3) 3rd argument in call

为0表示，它设置

It is 0 means, It set file status flag to zero. As Jean-Baptiste Yunès said in comment.

文件访问模式和文件创建标志将被忽略.这个命令 重置所有其他标志:无附加，无异步，无直接，无时间和 没有畅通无阻

file access mode and file creation flags are ignored. This command reset every other flags: no append, no async, no direct, no atime, and no nonblocking

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所以最后

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So finally

fcntl(fd, F_SETFL, 0)

此调用会将打开的文件描述符的file status标志设置为值0.

This call will set opened file desciptor's file status flag to value 0.

但是理想情况下，我们不应该更改文件状态标志. 最好的方法是先使用F_GETFL获取当前文件状态标志，然后在其中更改所需的位. 参见示例:

But idealy this way we should not change file status flag. Best way is to first get the current file status flag using F_GETFL and then just change required bit in that. See example:

如果要修改文件状态标志，则应使用F_GETFL获取当前标志并修改该值.不要以为这里列出的标志是唯一实现的标志；您的程序可能会从现在开始运行数年，然后可能会存在更多标志.例如，这是一个无需更改任何其他标志即可设置或清除标志O_NONBLOCK的函数:

If you want to modify the file status flags, you should get the current flags with F_GETFL and modify the value. Don’t assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag O_NONBLOCK without altering any other flags:

/* Set the O_NONBLOCK flag of desc if value is nonzero,
   or clear the flag if value is 0.
   Return 0 on success, or -1 on error with errno set. */

int
set_nonblock_flag (int desc, int value)
{
  int oldflags = fcntl (desc, F_GETFL, 0);
  /* If reading the flags failed, return error indication now. */
  if (oldflags == -1)
    return -1;
  /* Set just the flag we want to set. */
  if (value != 0)
    oldflags |= O_NONBLOCK;
  else
    oldflags &= ~O_NONBLOCK;
  /* Store modified flag word in the descriptor. */
  return fcntl (desc, F_SETFL, oldflags);
}

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