Re: sysread/syswrite and O_DIRECT alignment problem (align)

Using mmap() just to get an aligned buffer seems like a bit of a sledgehammer and might be causing problems (though I don't have swapped in the implications of that particular use of mmap). You could try a different approach:

my $bufsize= 4096;
my $align= 512;
my $buf= 'x' x ($align+$bufsize);
my $off= unpack( "J", pack "p", $buf ) % $align;
$off= $align - $off
    if  $off;
sysread( $FH, $buf, $bufsize, $off )
    or  ...;
my $data= substr( $buf, $off, $bufsize );
[download]

- tye

Comment on Re: sysread/syswrite and O_DIRECT alignment problem (align) Download Code

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Re^2: sysread/syswrite and O_DIRECT alignment problem (align) by fishnuts (Acolyte) on Nov 27, 2007 at 11:04 UTC
Tye, your solution worked perfectly, both for sysread and syswrite. How did you learn of this, and what have you used this for previously?	[reply]
Re^3: sysread/syswrite and O_DIRECT alignment problem (history) by tye (Sage) on Nov 27, 2007 at 17:01 UTC
How did you learn of this, and what have you used this for previously? I read the standard docs on pack and unpack and did experiments to learn the many aspects that aren't clearly explained. Acme::ESP uses parts of this. I've also used it several times to construct arguments to be passed through Win32::API. I also read the standard sysread docs and have used the 4th argument to efficiently append more to a buffer holding a scrolling window of data from a file. - tye	[reply]
Re^4: sysread/syswrite and O_DIRECT alignment problem (history) by fishnuts (Acolyte) on Nov 27, 2007 at 19:36 UTC
Awesome, thanks for the help!	[reply]
Re^2: sysread/syswrite and O_DIRECT alignment problem (align) by dk (Chaplain) on Nov 27, 2007 at 14:55 UTC
That doesn't work on my perl, where sizeof(UV) is 8 and sizeof(char*) is 4 :)	[reply]
Re^3: sysread/syswrite and O_DIRECT alignment problem (sizes) by tye (Sage) on Nov 27, 2007 at 16:22 UTC
Yeah, I almost mentioned that "J" might not be the right choice. Previously I'd used "L", but I think there are also situations where that doesn't work. I almost switched to writing code to detect the system's endianness and just pull out the least-significant byte, but the possibility of a system with mixed endianness deterred me. If I were to put this code someplace like a module I'd probably compare things like `length pack "p", "foo"` and `length pack "J", 0` to pick which format letter to use with unpack. - tye	[reply] [d/l] [select]