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Malcolm Beattie > Mmap > Mmap



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Mmap - uses mmap to map in a file as a perl variable


    use Mmap;

    mmap($foo, 0, PROT_READ, MAP_SHARED, FILEHANDLE) or die "mmap: $!";
    @tags = $foo =~ /<(.*?)>/g;
    munmap($foo) or die "munmap: $!";
    substr($bar, 1024, 11) = "Hello world";


The Mmap module lets you use mmap to map in a file as a perl variable rather than reading the file into dynamically allocated memory. It depends on your operating system supporting UNIX or POSIX.1b mmap, of course. You need to be careful how you use such a 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.


Maps LENGTH bytes of (the underlying contents of) FILEHANDLE into your address space, starting at offset OFFSET and makes VARIABLE refer to that memory. The OFFSET argument can be omitted in which case it defaults to zero. The LENGTH argument can be zero in which case a stat is done on FILEHANDLE and the size of the underlying file is used instead.

The PROTECTION argument should be some ORed combination of the constants PROT_READ, PROT_WRITE and PROT_EXEC or else PROT_NONE. The constants PROT_EXEC and PROT_NONE are unlikely to be useful here but are included for completeness.

The FLAGS argument must include either MAP_SHARED or MAP_PRIVATE (the latter is unlikely to be useful here). If your platform supports it, you may also use MAP_ANON or MAP_ANONYMOUS. If your platform supplies MAP_FILE as a non-zero constant (necessarily non-POSIX) then you should also include that in FLAGS. POSIX.1b does not specify MAP_FILE as a FLAG argument and most if not all versions of Unix have MAP_FILE as zero.

mmap returns 1 on success and undef on failure.


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.


The Mmap module exports the following constants into your namespace MAP_SHARED MAP_PRIVATE MAP_ANON MAP_ANONYMOUS MAP_FILE PROT_EXEC PROT_NONE PROT_READ PROT_WRITE

Of the constants beginning MAP_, only MAP_SHARED and MAP_PRIVATE are defined in POSIX.1b and only MAP_SHARED is likely to be useful.


Malcolm Beattie, 21 June 1996.

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