Re: O_DIRECT foolish question

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

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Just to complete the information, I am trying to read a file with 5
bytes, and here is the piece of code I am using:

    char *message;
    int fd = open("/var/tmp/testopen.txt", O_RDONLY|O_DIRECT);
    int len, pagesize = getpagesize();

    posix_memalign((void **)&message, pagesize, pagesize);
    if(fd < 0) {
        printf("Unable to open file, errno is %d.\n", errno);
    } else {
        if((len = read(fd, message, pagesize)) < 0) {
			perror("read");
        } else {
            printf("%d bytes read from file.\n", len);
	    printf("Message: %s", message);
        }
    }
    close(fd);

Thanks,

Bruno.

On Wed, 2003-02-12 at 18:02, Bruno Diniz de Paula wrote:
> 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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