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NAMESys::Mmap - uses mmap to map in a file as a Perl variableSYNOPSISuse Sys::Mmap; Sys::Mmap->new( my $str, 8192, 'structtest2.pl' ) or die $!; Sys::Mmap->new( $var, 8192 ) or die $!; mmap( $foo, 0, PROT_READ, MAP_SHARED, FILEHANDLE ) or die "mmap: $!"; @tags = $foo =~ /<(.*?)>/g; munmap($foo) or die "munmap: $!"; mmap( $bar, 8192, PROT_READ | PROT_WRITE, MAP_SHARED, FILEHANDLE ); substr( $bar, 1024, 11 ) = "Hello world"; mmap( $baz, 8192, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANON, STDOUT ); $addr = mmap( $baz, 8192, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANON, STDOUT ); Sys::Mmap::hardwire( $qux, $addr, 8192 ); DESCRIPTIONThe Sys::Mmap module uses the POSIX mmap <https://en.wikipedia.org/wiki/Mmap> call to map in a file as a Perl variable. Memory access by mmap may be shared between threads or forked processes, and may be a disc file that has been mapped into memory. Sys::Mmap depends on your operating system supporting UNIX or POSIX.1b mmap, of course.Note that PerlIO now defines a ":mmap" tag and presents mmap'd files as regular files, if that is your cup of joe. Several processes may share one copy of the file or string, saving memory, and concurrently making changes to portions of the file or string. When not used with a file, it is an alternative to SysV shared memory. Unlike SysV shared memory, there are no arbitrary size limits on the shared memory area, and sparse memory usage is handled optimally on most modern UNIX implementations. Using the "new()" method provides a "tie()"'d interface to "mmap()" that allows you to use the variable as a normal variable. If a filename is provided, the file is opened and mapped in. If the file is smaller than the length provided, the file is grown to that length. If no filename is provided, anonymous shared inheritable memory is used. Assigning to the variable will replace a section in the file corresponding to the length of the variable, leaving the remainder of the file intact and unmodified. Using "substr()" allows you to access the file at an offset, and does not place any requirements on the length argument to "substr()" or the length of the variable being inserted, provided it does not exceed the length of the memory region. This protects you from the pathological cases involved in using "mmap()" directly, documented below. When calling "mmap()" or "hardwire()" directly, you need to be careful how you use the variable. Some programming constructs may create copies of a string which, while unimportant for smallish strings, are far less welcome if you're mapping in a file which is a few gigabytes big. If you use "PROT_WRITE" and attempt to write to the file via the variable you need to be even more careful. One of the few ways in which you can safely write to the string in-place is by using "substr()" as an lvalue and ensuring that the part of the string that you replace is exactly the same length. Other functions will allocate other storage for the variable, and it will no longer overlay the mapped in file.
BUGSScott Walters doesn't know XS, and is just winging it. There must be a better way to tell Perl not to reallocate a variable in memory...The "tie()" interface makes writing to a substring of the variable much less efficient. One user cited his application running 10-20 times slower when "Sys::Mmap->new()" is used than when "mmap()" is called directly. Malcolm Beattie has not reviewed Scott's work and is not responsible for any bugs, errors, omissions, stylistic failings, importabilities, or design flaws in this version of the code. There should be a tied interface to "hardwire()" as well. Scott Walter's spelling is awful. "hardwire()" will segfault Perl if the "mmap()" area it was referring to is "munmap()"'d out from under it. "munmap()" will segfault Perl if the variable was not successfully "mmap()"'d previously, or if it has since been reallocated by Perl. AUTHORCowboyTim added support for MAP_NORESERVE, MAP_HUGETLB, MAP_HUGE_2MB, and MAP_HUGE_1GB. Thanks CowboyTim!Todd Rinaldo cleaned up code, modernized again, and merged in many fixes, 2010-2011. Scott Walters updated for Perl 5.6.x, additions, 2002. Malcolm Beattie, 21 June 1996.
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