| Subject: | The buffer seems to be a copy of the mapped memory |
The test program below (which is written for Linux) mmaps a buffer to a
file ("/tmp/testfile", can be initialized whith arbitrary content), then
the block is exited, although a reference to the buffer is retained.
Before the block is exited, the "testfile" appears in /proc/$$/maps
(which should show all files that are currently mmapped into the
process). After exiting the block, "testfile" does not appear any more
in the maps file. However, the buffer can still be accessed, and the
md5 sum is still the same, leading to the conclusion that it is only a
copy of the mapped data.
Interestingly, if we create a second reference to the buffer (the
commented line), the "testfile" still appears in maps.
After changing the buffer in the last line of the script, the file on
disk does not change, which reinforces my theory that the buffer is only
a copy.
#!/usr/bin/perl
use strict;
use warnings;
use Sys::Mmap::Simple qw(map_handle map_file);
use Digest::MD5 qw(md5_hex);
my $this = {};
open my $fh, '+<', '/tmp/testfile' or die $!;
$this->{buffer} = \do {
map_handle(my $buf, $fh, '+<') or die $!;
warn md5_hex($buf);
warn "first grep:\n";
system("grep testfile /proc/$$/maps >/dev/tty");
warn "----\n";
# $::x = \$buf;
$buf;
};
warn "second grep:\n";
system("grep testfile /proc/$$/maps >/dev/tty");
warn "----\n";
warn md5_hex ${$this->{buffer}};
substr(${$this->{buffer}}, 0, 1) = 'x';
Now here is the output:
shell$ cat /tmp/testfile
123
shell$ perl map.pl
ba1f2511fc30423bdbb183fe33f3dd0f at map.pl line 15.
first grep:
b7fd1000-b7fd2000 rw-s 00000000 00:0f 147712 /tmp/testfile
----
second grep:
----
ba1f2511fc30423bdbb183fe33f3dd0f at map.pl line 26.
shell$ cat /tmp/testfile
123
I'd have expected to see "testfile" in the output after "second grep",
and I'd have expected that the test file's content would be "x23" after
the script run.
The test suite runs fine here, however I've noticed that there are no
tests that affirm that the file really changes on disk if the mmapped
buffer is modified.