13 Feb 2020 18:36:50 UTC
- Distribution: Sys-Mmap
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Sys::Mmap - uses mmap to map in a file as a Perl variable
use 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 );
The Sys::Mmap module uses the POSIX 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
:mmaptag 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.
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.
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_WRITEand 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.
LENGTHbytes of (the contents of)
OPTIONALFILENAMEis provided, otherwise uses anonymous, shared inheritable memory. This memory region is inherited by any
VARIABLEwill now refer to the contents of that file. Any change to
VARIABLEwill make an identical change to the file. If
LENGTHis zero and a file is specified, the current length of the file will be used. If
LENGTHis larger then the file, and
OPTIONALFILENAMEis provided, the file is grown to that length before being mapped. This is the preferred interface, as it requires much less caution in handling the variable.
VARIABLEwill be tied into the "Sys::Mmap" package, and
mmap()will be called for you.
VARIABLEwill overwrite the beginning of the file for a length of the value being assigned in. The rest of the file or memory region after that point will be left intact. You may use
substr()to assign at a given position:
substr(VARIABLE, POSITION, LENGTH) = NEWVALUE
- mmap(VARIABLE, LENGTH, PROTECTION, FLAGS, FILEHANDLE, OFFSET)
LENGTHbytes of (the underlying contents of)
FILEHANDLEinto your address space, starting at offset
VARIABLErefer to that memory. The
OFFSETargument can be omitted in which case it defaults to zero. The
LENGTHargument can be zero in which case a stat is done on
FILEHANDLEand the size of the underlying file is used instead.
PROTECTIONargument should be some ORed combination of the constants
PROT_EXEC, or else
PROT_NONE. The constants
PROT_NONEare unlikely to be useful here but are included for completeness.
FLAGSargument must include either
MAP_PRIVATE(the latter is unlikely to be useful here). If your platform supports it, you may also use
MAP_ANONYMOUS. If your platform supplies
MAP_FILEas a non-zero constant (necessarily non-POSIX) then you should also include that in
FLAGS. POSIX.1b does not specify
FLAGargument and most if not all versions of Unix have
undefon failure, and the address in memory where the variable was mapped to on success.
Unmaps the part of your address space which was previously mapped in with a call to
mmap(VARIABLE, ...)and makes VARIABLE become undefined.
munmap returns 1 on success and undef on failure.
- hardwire(VARIABLE, ADDRESS, LENGTH)
Specifies the address in memory of a variable, possibly within a region you've
mmap()ed another variable to. You must use the same precautions to keep the variable from being reallocated, and use
substr()with an exact length. If you
munmap()a region that a
hardwire()ed variable lives in, the
hardwire()ed variable will not automatically be
undefed. You must do this manually.
The Sys::Mmap module exports the following constants into your namespace:
MAP_SHARED MAP_PRIVATE MAP_ANON MAP_ANONYMOUS MAP_FILE MAP_NORESERVE MAP_POPULATE MAP_HUGETLB MAP_HUGE_2MB MAP_HUGE_1GB PROT_EXEC PROT_NONE PROT_READ PROT_WRITE
Of the constants beginning with
MAP_PRIVATEare defined in POSIX.1b and only
MAP_SHAREDis likely to be useful.
Scott 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...
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
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.
CowboyTim 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.