我如何测试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

我有一些子程序，像 myWrite（$ fileName，\ @ data）。 myWrite（）以某种方式打开文件并写出数据。我想修改 myWrite ，这样我就可以像上面的或那样用文件句柄作为第一个参数来调用它。 （这个修改的主要原因是把文件的打开委托给调用脚本而不是模块，如果有更好的方法来告诉IO子程序在哪里写，我很乐意听到。 ）

为了做到这一点，我必须测试第一个输入变量是否是一个文件句柄。我想通过阅读这个问题。

现在我的问题是：我也想测试是否可以写入这个文件句柄。我不知道如何做到这一点。

这是我想要做的：

  sub myWrite {
 my（$ writeTo，$ data）= @_; 
 my $ fh; 
 if（isFilehandle（$ writeTo））{＃我可以做到这一点
死你是一个不道德的人\
除非（canWriteTo（$ writeTo））; ＃但是我该怎么做呢？ 
 $ fh = $ writeTo; 
} else {
打开$ fh，>，$ writeTo; 
} 
 ... 
} 
  

我需要知道是否我可以写入文件句柄，虽然看到一些通用的解决方案会告诉你是否用>>或<打开了文件句柄，或者如果没有打开等等。

（注意是相关的，但似乎并没有回答我的问题。）

解决方案

  
 
打开A，'<'，'/ some / file'; 
打开B，'>'，'/ some / other-file'; 
 
 {
本地$！ = 0; 
 my $ n = syswrite A，; 
＃结果：$ n是undef，$！是错误的文件描述符
} 
 {
 local $！ = 0; 
 my $ n = syswrite B，; 
＃结果：$ n是0，$！是

  

fcntl 也很有希望。你的里程可能会有所不同，但类似的情况可能是正确的：

pre $ 使用Fcntl;
$ flags = fcntl HANDLE，F_GETFL，0; （$ flags& O_ACCMODE）&（O_WRONLY | O_RDWR））{
printHANDLE是可写的... \\\

} $ bGET FLags
$ b

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.)

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.

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.

Here's what I want to do:

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

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.

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

解决方案

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 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 ...\n"
}

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