为什么打开undef不会失败？

Question

此代码如我所料那样死亡：

use strict;
use warnings;

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

但这不是：

use strict;
use warnings;

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

这是怎么回事？

Answer 1

open 函数有很多小怪癖，这就是其中之一：

作为一种特殊情况，具有读/写模式的三参数形式和 第三个参数是“undef”：

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

打开一个匿名临时文件的文件句柄。也使用“+

虽然，正如 cmets 中的 ysth 所指出的，文档强烈建议这应该只发生在“++”模式下。我相信这是实现行为的代码。它不检查模式。我不知道这是否是一个错误，但会在与 P5P 交谈后反馈。

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);
        }
    }

显然，文档是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