[英]In Perl, why can't you use __DATA__ as a seekable filehandle?
Reading from DATA via the typical slurp works. 通过典型的slurp作品从DATA读取。 Trying to use DATA as a filehandle on which I can do a seek does not work. 尝试使用DATA作为我可以进行搜索的文件句柄是行不通的。 Is anyone able to point me the to the obvious mistake I must be making? 有人能够指出我必须犯的明显错误吗?
Code: 码:
#!/usr/bin/env perl
use strict;
use warnings;
if ($ARGV[0] eq 'seek' ) {
my $log_fh = \*DATA;
$log_fh->seek(64,0);
print "\n-- 64 --\n",join ("", <$log_fh> );
} else {
while (<DATA>) {
print $_;
}
}
exit;
__DATA__
01234567890123456789
1234567890123456789
1234567890123456789
12
X <- That X is the 64th char in
this file.
Y <- That Y is the 106th char in this file.
junk
more junk.
bye!
$ perl file_from_data.pl slurp
01234567890123456789
1234567890123456789
1234567890123456789
12
X <- That X is the 64th char in
this file.
Y <- That Y is the 106th char in this file.
junk
more junk.
bye!
Running the while() loop: 运行while()循环:
$ perl file_from_data.pl slurp
01234567890123456789
1234567890123456789
1234567890123456789
12
X <- That X is the 64th char in
this file.
Y <- That Y is the 106th char in this file.
junk
more junk.
bye!
Running the seek(), it appears to not start at DATA but the start of the script: 运行seek(),它似乎不是从DATA开始,而是脚本的开始:
$ perl file_from_data.pl seek
-- 64 --
'seek' ) {
my $log_fh = \*DATA;
$log_fh->seek(64,0);
print "\n-- 64 --\n",join ("", <$log_fh> );
} else {
while (<DATA>) {
print $_;
}
}
exit;
__DATA__
01234567890123456789
1234567890123456789
1234567890123456789
12
X <- That X is the 64th char in
this file.
Y <- That Y is the 106th char in this file.
junk
more junk.
bye!
This is an old Perl: 这是一个旧的Perl:
$ perl -v
This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-
thread-multi
Running the seek(), it appears to not start at DATA but the start of the script 运行seek(),它似乎不是从DATA开始,而是从脚本的开始
I don't think you're making any mistake at all. 我认为你根本没有犯任何错误。 That's exactly what happens. 这正是发生的事情。 DATA
is a filehandle that is open on your source file. DATA
是在源文件上打开的文件句柄。 Before your first read()
from that filehandle, the file pointer is positioned immediately after the __DATA__
token in the file. 在您从该文件句柄第一次read()
之前,文件指针紧跟在文件中的__DATA__
标记之后。 But you can use seek()
to move the file pointer to any position at all in the file. 但是您可以使用seek()
将文件指针移动到文件中的任何位置。
I guess it would be harder to implement a "special case" filehandle that wasn't able to move back before its initial position. 我想要实现一个无法在其初始位置之前移回的“特殊情况”文件句柄会更难。
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