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    SubjectRe: O_DIRECT foolish question
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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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