perl - 这样的代码的目的是什么？

Question

253:        my $sel = select(FOUT);
254:        $| = 1;                             # for DB::OUT
255:        select($sel);

对我来说看起来很奇怪，在 Term::ReadLine 中发现模块。

Answer 1

Writing to STDOUT (or any other output filehandle) is buffered by default. To ask Perl to flush immediately after each write or print command, set the special variable $| to 1.

http://www.perlhowto.com/disable_output_buffering

编辑:进一步解释:

my $sel = select(FOUT);

FOUT是一个文件句柄，使用 select使它成为 default file handle以便任何使用 default file handle 的操作现在将使用 FOUT .例如 print "moo"相当于 print FOUT "moo" .
select的返回值是以前的 default file handle ，即标准输出。

$| = 1;

此命令禁用 default file handle 上的输出缓冲。 , 因为句柄是 FOUT为 FOUT 禁用输出缓冲.

select($sel);

现在我们带回之前的 default file handle ，即标准输出，所以 print命令等按预期工作。

编辑#2:文件句柄的进一步解释:

假设您有一系列文件句柄 STDOUT , FILE_ONE , FILE_TWO , SOCKET_ONE , 和 SOCKET_TWO .你想设置 FILE_ONE和 SOCKET_TWO没有输出缓冲。

# On startup Perl effectively does the following:
# select(STDOUT);
my $sel = select(FILE_ONE);
# $sel is now STDOUT
$| = 1;
select(SOCKET_TWO);
$| = 1;
# bring back STDOUT
select($sel);

现在让我们来看看神奇的 default file handle 会发生什么.

print "HELLO\n";
# equivalent to: print STDOUT "HELLO\n";
my $sel = select(FILE_ONE);
# sets `default file handle` to FILE_ONE
print "HELLO\n";
# equivalent to: print FILE_ONE "HELLO\n";
$| = 1;
# disables output buffering on handle FILE_ONE
select(SOCKET_TWO)
# sets `default file handle` to SOCKET_TWO
print "HELLO\n";
# equivalent to: print SOCKET_TWO "HELLO\n";
$| = 1;
# disables output buffering on handle SOCKET+TWO
select($sel);
# sets `default file handle` to STDOUT

或者让我们发明一些新变量:

$FH
# let this be the `default file handle`

让我们发明一个新功能:

sub disable_output_buffer ($file_handle) {
# magic occurs here
}

现在让我们使用这个新的文件句柄和函数重写之前的代码。

# print "HELLO\n";
my $FH = STDOUT;
print $FH "HELLO\n"           # print STDOUT "HELLO\n"

# my $sel = select(FILE_ONE);
my $sel = $FH;
$FH = FILE_ONE;

# print "HELLO\n";
print $FH "HELLO\n";          # print FILE_ONE "HELLO\n"

# $| = 1
disable_output_buffer ($FH);  # disable_output_buffer (FILE_ONE)

# select(SOCKET_TWO);
$FH = SOCKET_TWO;

# print "HELLO\n";
print $FH "HELLO\n";          # print SOCKET_TWO "HELLO\n"

# $| = 1
disable_output_buffer ($FH);  # disable_output_buffer (SOCKET_TWO)

# select($sel);
$FH = $sel;