Harry Putnam <[EMAIL PROTECTED]> wrote:
> Steve Grazzini <[EMAIL PROTECTED]> writes:
>> Harry Putnam <[EMAIL PROTECTED]> wrote:
>>>
>>> I've seen that `_' crop up before
>>> I don't understand what this means.
>>
>> It's documented in perlfunc:
>
> Yes, I saw it there too. I must be having a particularly dense
> time of it, but I still am missing what is actually in _
Nothing's in it. It's just a filehandle.
If you call stat() or use a file test operator on an ordinary
filehandle, Perl uses the fstat() system call:
if (-t STDOUT) { # connected to terminal
If you call stat() or use a file test operator on the special _
filehandle, perl *skips the system call* and returns the results
of the previous stat() (or lstat() or fstat()) system call,
which have been squirreled away somewhere.
There's nothing in the _ filehandle; it's just a signal to perl
that we want to reuse those cached results.
> The previous content of the stat structure is what? (in plain
> english) Does it mean the previous values of the 13 elements
> produced by stat?
Something like that. The 'stat structure' is the big C struct
described in the stat(2) manpage. The perl interpreter has one
of these as a sort of global variable.
When you do "stat $file", perl makes the stat system call,
which populates this struct. The values in the structure are
left there afterward, and when you do "stat _", perl skips
the system call and reuses whatever is already in the struct.
There are two principles at work
(1) system calls are relatively expensive: so we want to
cache the results of stat() if we need to do more than
one file test on the same file/filehandle.
(2) the file test operators (e.g. '-f _') are much more
convenient than the S_ISWHATEVER macros or their equivalent
bit-shifting: so we'd prefer to let perl do the caching for
us internally.
--
Steve
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