如何测试我是否可以写入文件句柄? [英] How can I test if I can write to a filehandle?

问题描述

我有一些像这样调用的子程序 myWrite($fileName, @data).myWrite() 打开文件并以某种方式写出数据.我想修改 myWrite 以便我可以像上面一样调用它或，并将文件句柄作为第一个参数.(这种修改的主要原因是将文件的打开委托给调用脚本而不是模块.如果有更好的解决方案来告诉 IO 子例程在哪里写，我很高兴听到它.)

I have some subroutines that I call like this myWrite($fileName, @data). myWrite() opens the file and writes out the data in some way. I want to modify myWrite so that I can call it as above or with a filehandle as the first argument. (The main reason for this modification is to delegate the opening of the file to the calling script rather than the module. If there is a better solution for how to tell an IO subroutine where to write, i'd be glad to hear it.)

为了做到这一点，我必须测试第一个输入变量是否是文件句柄.我通过阅读 这个问题.

In order to do this, I must test whether the first input var is a filehandle. I figured out how to do that by reading this question.

现在是我的问题:我还想测试我是否可以写入这个文件句柄.我不知道该怎么做.

Now here's my question: I also want to test whether I can write to this filehandle. I can't figure out how to do that.

这是我想要做的:

sub myWrite {
  my ($writeTo, $data) = @_;
  my $fh;
  if (isFilehandle($writeTo)) { # i can do this
    die "you're an immoral person
" 
      unless (canWriteTo($writeTo)); # but how do I do this?
    $fh = $writeTo;
  } else {
    open $fh, ">", $writeTo;
  }
  ...
}

我只需要知道我是否可以写入文件句柄，尽管很高兴看到一些通用解决方案告诉您文件句柄是用>>"还是<"打开的，或者如果它没有打开等

All I need to know is if I can write to the filehandle, though it would be nice to see some general solution that tells you whether you're filehandle was opened with ">>" or "<", or if it isn't open, etc.

(注意这个问题 是相关的，但似乎没有回答我的问题.)

(Note that this question is related but doesn't seem to answer my question.)

推荐答案

仍在试验这个，但也许您可以尝试对文件句柄进行零字节系统写入并检查错误:

Still experimenting with this, but maybe you can try a zero-byte syswrite to a filehandle and check for errors:

open A, '<', '/some/file';
open B, '>', '/some/other-file';

{
    local $! = 0;
    my $n = syswrite A, "";
    # result: $n is undef, $! is "Bad file descriptor"
}
{
    local $! = 0;
    my $n = syswrite B, "";
    # result: $n is 0, $! is ""
}

fcntl 看起来也很有希望.您的里程可能会有所不同，但这样的事情可能走在正确的轨道上:

fcntl looks promising too. Your mileage may vary, but something like this could be on the right track:

use Fcntl;
$flags = fcntl HANDLE, F_GETFL, 0;  # "GET FLags"
if (  ($flags & O_ACCMODE) & (O_WRONLY|O_RDWR) ) {
    print "HANDLE is writeable ...
"
}

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