爲什麼打開一個undef不會失敗？

Question

此代碼爲死去我希望它：

use strict; 
use warnings; 

open my $fh, "<", "" or die $!; 

但這並不：

use strict; 
use warnings; 

open my $fh, "<", undef or die $!; 

這到底是怎麼回事？

Answer 1

的open功能有很多小怪癖的，這就是其中之一：

作爲一個特殊的情況下，三個參數的形式與讀/寫模式和 第三個參數是「民主基金」：

open(my $tmp, "+>", undef) or die ... 

打開文件句柄到匿名臨時文件。同樣使用「+ <」 適用於對稱性，但您應該首先考慮將臨時文件寫入 。您需要尋求（）來完成 閱讀。

儘管正如註釋中提到的那樣，但文檔強烈建議這隻應發生在「+ <」和「> +」模式下。我相信這是實現行爲的代碼。它不檢查模式。我不知道這是否是一個錯誤，但會在與P5P交談後報告。

PerlIO * 
PerlIO_openn(pTHX_ const char *layers, const char *mode, int fd, 
      int imode, int perm, PerlIO *f, int narg, SV **args) 
{ 
    if (!f && narg == 1 && *args == &PL_sv_undef) { 
     if ((f = PerlIO_tmpfile())) { 
      if (!layers || !*layers) 
       layers = Perl_PerlIO_context_layers(aTHX_ mode); 
      if (layers && *layers) 
       PerlIO_apply_layers(aTHX_ f, mode, layers); 
     } 
    } 

顯然，文檔是fixed in blead perl in November：

diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod 
index 18bb4654e1..1e32cca6dd 100644 
--- a/pod/perlfunc.pod 
+++ b/pod/perlfunc.pod 
@@ -4405,9 +4405,9 @@ argument being L<C<undef>|/undef EXPR>: 

    open(my $tmp, "+>", undef) or die ... 

-opens a filehandle to an anonymous temporary file. Also using C<< +< >> 
-works for symmetry, but you really should consider writing something 
-to the temporary file first. You will need to 
+opens a filehandle to a newly created empty anonymous temporary file. 
+(This happens under any mode, which makes C<< +> >> the only useful and 
+sensible mode to use.) You will need to 
L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading. 

Perl is built using PerlIO by default. Unless you've