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Five Minute Lightning Talk: Perl's syscall Function

#16

mmap2

Almost identical to mmap, but Linux specific
Last argument is specified as multiples of system page size instead of bytes

    my $addr = syscall(&SYS_mmap2, 0, $size,
            &PROT_READ, &MAP_SHARED, fileno F, 0);

    die "$!\n" unless ($addr > 0);
    print "It worked!\n";

Initially it seems to work!

    [mct@ellesmere ~]$ strace -o out ./mmap.pl
    It worked!

...but analysis of the strace file shows otherwise:

    [mct@ellesmere ~]$ tail -4 out | head -2
    mmap2(NULL, 8483022, PROT_READ, MAP_SHARED, 3, 0xbffff0a8) = 0x401ea000
    write(1, "It worked!\n", 11)            = 11

Why is the last argument 0xbffff0a8 rather than 0?
I haven't yet had time to gdb Perl to see what's really happening