Re: O_DIRECT foolish question

[Bruno Diniz de Paula <diniz@cs.rutgers.edu>]

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On Wed, 2003-02-12 at 17:42, Chris Wedgwood wrote:
> On Wed, Feb 12, 2003 at 05:29:52PM -0500, Bruno Diniz de Paula wrote:
> 
> > But I am using multiples of page size in both buffer alignment and
> > buffer size (2nd and 3rd parameters of read).  The issue is that
> > when I try to read files with sizes that are NOT multiples of block
> > size (and therefore also not multiples of page size), the read
> > syscall returns 0, with no errors.
> 
> What filesystem?

ext2.

> 
> Can you send an strace of this occurring?

execve("./testopen", ["./testopen"], [/* 30 vars */]) = 0
uname({sys="Linux", node="urca", ...})  = 0
brk(0)                                  = 0x80497fc
open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or
directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=57677, ...}) = 0
old_mmap(NULL, 57677, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40012000
close(3)                                = 0
open("/lib/libc.so.6", O_RDONLY)        = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0]Z\1\000"...,
1024) = 1024
fstat64(3, {st_mode=S_IFREG|0755, st_size=1102984, ...}) = 0
old_mmap(NULL, 1112740, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) =
0x40021000
mprotect(0x40129000, 31396, PROT_NONE)  = 0
old_mmap(0x40129000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED,
3, 0x107000) = 0x40129000
old_mmap(0x4012f000, 6820, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x4012f000
close(3)                                = 0
old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
-1, 0) = 0x40131000
munmap(0x40012000, 57677)               = 0
open("/var/tmp/testopen.txt", O_RDONLY|O_DIRECT) = 3
brk(0)                                  = 0x80497fc
brk(0x804c7fc)                          = 0x804c7fc
brk(0)                                  = 0x804c7fc
brk(0x804d000)                          = 0x804d000
read(3, "", 4096)                       = 0
fstat64(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 4), ...}) = 0
old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
-1, 0) = 0x40012000
write(1, "0 bytes read from file.\n", 240 bytes read from file.
) = 24
close(3)                                = 0
write(1, "Message: ", 9Message: )                = 9
munmap(0x40012000, 4096)                = 0
exit_group(0)                           = ?

Thanks a lot,

Bruno.

> 
> > So the question remains, am I able to read just files whose size is
> > a multiple of block size?
> 
> No.
> 
> You ideally should be able to read any length file with O_DIRECT.
> Even a 1-byte file.
> 
> 
> 
>   --cw
-- 
Bruno Diniz de Paula <diniz@cs.rutgers.edu>
Rutgers University

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