只允许将一个文件重定向到标准输入 [英] Only allow one file to be redirected to stdin

问题描述

我目前正在编写具有用法的C ++程序:

I am currently writing a C++ program with usage:

[prog]  - - - - - - - - - - - - read from stdin
[prog] [filename]   - - - - - - will test and open file
[prog] < [filename]   - - - - - will redirect the filename to stdin

我编写了捕获太多参数的代码，并且一切正常，除了用户尝试

I have written code to catch too many arguments, and everything works, except that if a user tries

[prog] < [filename] [filename]

它将打开第二个文件名.

it will open the second filename.

如何防止这种情况发生.感谢您的考虑.

How do I prevent this case from happening. Thanks for your consideration.

推荐答案

进程没有什么可以看成权威地断定shell是否显式重定向了进程的标准输入而不是默认值.

There's nothing that a process can look at to authoritatively conclude whether or not the shell explicitly redirected the process's standard input away from the default.

但是，当在POSIX平台上运行时，进程可以做出非常有根据的猜测，其中

But a process can make a very educated guess when running on a POSIX platform, where isatty() returns an indication whether the given file descriptor is attached to an interactive terminal. So, you can check if isatty(0) (standard input file descriptor #0); if so this means that the process's standard input is still attached to interactive keyboard input, and has not been redirected.

这不是100％可靠的.仍然可以通过精心制作的歌舞表演程序，并使用伪tty设备将管道连接到与交互式终端无法区分的流程标准输入，但是输入确实来自其他来源，但这很少见，对于典型的花园应用而言，isatty()就足够了.因此，您的main()可以检查isatty(0)，如果是，则很好地表明输入尚未重定向.

This is not 100% foolproof. It is still possible to go through an elaborate song-and-dance routine, and using a pseudo-tty device attach a pipe to a process's standard input that's indistinguishable from an interactive terminal, to the process, but the input really comes from some other source, but this is very rare, and for your typical garden-variety application, isatty() should be sufficient. So, your main() can check isatty(0), and if so it's a very good indication that the input has not been redirected.

但是，请注意，如果isatty(0)不是true，则可能会重定向标准输入，但是可能需要进行额外的侦查才能确定它是管道，纯文件还是其他内容.

Note, however, if isatty(0) is not true, the standard input is likely redirected, but additional sleuthing will be required to figure out if it's a pipe, or a plain file, or something else, perhaps.

因此，您的总体游戏计划是:检查argv是否指定文件名.如果是，则为isatty(0)，那么您将要处理两个可能的输入文件.

So, your overall gameplan here is: check if your argv specifies a filename. If so, if isatty(0), then you've got two likely input files to deal with.

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