Sys-Mmap reviews


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Sys-Mmap (0.16) **

Although this module mostly does what it claims to, it is too low-level to be used reliably from perl. Instead, check out File::Map.

The biggest issue with Sys::Mmap is that it is difficult to ensure that mappings are always unmapped when you are finished with them:

use Sys::Mmap;

eval {

open(my $fh, "/etc/passwd") || die;

mmap(my $str, 0, PROT_READ, MAP_SHARED, $fh) || die;

die "Something bad happened!";




## Mapping still exists (but $fh was closed)

With File::Map your mappings will be unmapped automatically:

use File::Map;

eval {

File::Map::map_file my $str, "/etc/passwd", '<';

die "Something bad happened!";


## Mapping has been released

Other issues with Sys::Mmap:

* It is not as portable as File::Map which also supports windows.

* The synopsis seems to be wrong. You have to do "new Sys::Mmap", not "new Mmap". But don't use this interface since it doesn't seem to be possible to clean up mappings after they are no longer needed.

* There is no way to create MAP_FIXED mappings (although you generally don't need this).

Although not an issue with this module, you should be aware that there is a bug in perl versions before 5.17.5 where regular expressions can read one byte past the end of a string. This can segfault your perl process if you create mappings that are exact multiples of your page size (and your OS doesn't create adjacent mappings):

$ uname -a

OpenBSD saturn 5.1 GENERIC#181 amd64

$ perl -E 'say $^V'


$ perl -MSys::Mmap -e 'open($fh, "/dev/zero"); mmap($str, 4096, PROT_READ, MAP_ANON, $fh); $str =~ /.$/;'

Segmentation fault (core dumped)

Sys-Mmap (0.13) *****

Simple, works as advertised, wish it were core. It's great for mapping in a huge chunk of text and screaming through it with a regex. I haven't benchmarked it, but this is almost certainly much faster than slurping a file when you need read-only access.