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List:       linux-aio
Subject:    AIO O_DIRECT read bug on last block.
From:       Drangon Zhou <drangon.zhou () gmail ! com>
Date:       2005-07-29 11:22:09
Message-ID: 1305ed680507290422675dbb8 () mail ! gmail ! com
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Hi,

I use linux AIO with O_DIRECT to read a file, when I read the last block,
it doesn't return the actual size, but return some size that 512 byte align.

for example, I have a file a.txt, size is 300 bytes, if I use normal read(),
it return 300, this is ok, if I use io_submit() and io_getevents(), it
return 512,
Is this a bug ??

-----------------------------
int main( int argc, char * argv[] )
{
  char * buffer;
  int ret;
  int fd = open( "a.txt", O_RDONLY | O_DIRECT );
  posix_memalign( &buffer, 4096, 4096 );

  // <1> use normal disk read, this is ok
  ret = read( fd, buffer, 4096 );
  printf( "ret = %d\n", ret ); // ret = 300, this ok.

  // <2> use AIO disk read, it seems error.
  struct iocb myiocb;
  struct iocb * cb = &myiocb;
  memset( &myiocb, 0, sizeof(myiocb) );
  cb->data = 0;
  cb->key = 0;
  cb->aio_lio_opcode = IO_CMD_PREAD;
  cb->aio_reqprio = 0; 
  cb->aio_fildes = fd; 
  cb->u.c.buf = buffer;
  cb->u.c.nbytes = 4096;
  cb->u.c.offset = 0;
    
  int num = 0;
  struct io_event ie;
  struct timespec ts = {0,0};
    
  ret = io_submit( ioctx, 1, &cb );
  // a foolish loop, only to see if this is really an AIO, 
  while( ++num ) {
    ret = io_getevents( ioctx, 1, 1, &ie, &ts );
    if( ret == 1 )
        break; 
  }       
  printf( "ret %ld, ie.res ); // it return 512 ?? bug ?? should be 300
}
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