How can you flush a write using a file descriptor

Question

It turns out this whole misunderstanding of the open   versus fopen   stems from a buggy I2C driver in the Linux 2 6 14 kernel on an ARM   Backporting a working bit bashed driver solved the root cause of the problem I was trying to address here    I m trying to figure out an issue with a serial device driver in Linux  I2C    It appears that by adding timed OS pauses  sleeps  between writes and reads on the device things work      much  better       Aside  The nature of I2C is that each byte read or written by the master is acknowledged by the device on the other end of the wire  slave  - the pauses improving things encourage me to think of the driver as working asynchronously - something that I can t reconcile with how the bus works   Anyhoo       I d either like to flush the write to be sure  rather than using fixed duration pause   or somehow test that the write read transaction has completed in an multi-threaded friendly way     The trouble with using fflush fd   is that it requires  fd  to be stream pointer  not a file descriptor  i e   FILE   fd   fopen  filename   r            do read and writes fflush fd     My problem is that I require the use of the ioctl    which doesn t use a stream pointer  i e   int fd   open  filename  O RDWR   ioctl fd         Suggestions

User · Accepted Answer

You have two choices    Use fileno   to obtain the file descriptor associated with the stdio stream pointer Don t use  lt stdio h gt  at all  that way you don t need to worry about flush either - all writes will go to the device immediately  and for character devices the write   call won t even return until the lower-level IO has completed  in theory     For device-level IO I d say it s pretty unusual to use stdio   I d strongly recommend using the lower-level open    read   and write   functions instead  based on your later reply    int fd   open   dev i2c   O RDWR   ioctl fd  IOCTL COMMAND  args   write fd  buf  length

User · Answer

If you want to go the other way round  associate FILE  with existing file descriptor   use fdopen                                                                FDOPEN P   NAME         fdopen - associate a stream with a file descriptor  SYNOPSIS          include  lt stdio h gt          FILE  fdopen int fildes  const char  mode

User · Answer

Have you tried disabling buffering   setvbuf fd  NULL   IONBF  0

User · Answer

If you want to go the other way round  associate FILE  with existing file descriptor   use fdopen                                                                FDOPEN P   NAME         fdopen - associate a stream with a file descriptor  SYNOPSIS          include  lt stdio h gt          FILE  fdopen int fildes  const char  mode

User · Answer

It sounds like what you are looking for is the fsync   function  or fdatasync      or you could use the O SYNC flag in your open   call

User · Answer

fflush   only flushes the buffering added by the stdio fopen   layer  as managed by the FILE   object  The underlying file itself  as seen by the kernel  is not buffered at this level  This means that writes that bypass the FILE   layer  using fileno   and a raw write    are also not buffered in a way that fflush   would flush   As others have pointed out  try not mixing the two  If you need to use  raw  I O functions such as ioctl    then open   the file yourself directly  without using fopen lt    and friends from stdio

User · Answer

If you want to go the other way round  associate FILE  with existing file descriptor   use fdopen                                                                FDOPEN P   NAME         fdopen - associate a stream with a file descriptor  SYNOPSIS          include  lt stdio h gt          FILE  fdopen int fildes  const char  mode

User · Answer

I think what you are looking for may be  int fsync int fd     or  int fdatasync int fd     fsync will flush the file from kernel buffer to the disk  fdatasync will also do except for the meta data

User · Answer

fflush   only flushes the buffering added by the stdio fopen   layer  as managed by the FILE   object  The underlying file itself  as seen by the kernel  is not buffered at this level  This means that writes that bypass the FILE   layer  using fileno   and a raw write    are also not buffered in a way that fflush   would flush   As others have pointed out  try not mixing the two  If you need to use  raw  I O functions such as ioctl    then open   the file yourself directly  without using fopen lt    and friends from stdio

User · Answer

fflush   only flushes the buffering added by the stdio fopen   layer  as managed by the FILE   object  The underlying file itself  as seen by the kernel  is not buffered at this level  This means that writes that bypass the FILE   layer  using fileno   and a raw write    are also not buffered in a way that fflush   would flush   As others have pointed out  try not mixing the two  If you need to use  raw  I O functions such as ioctl    then open   the file yourself directly  without using fopen lt    and friends from stdio

User · Answer

I think what you are looking for may be  int fsync int fd     or  int fdatasync int fd     fsync will flush the file from kernel buffer to the disk  fdatasync will also do except for the meta data

User · Answer

It sounds like what you are looking for is the fsync   function  or fdatasync      or you could use the O SYNC flag in your open   call

User · Answer

Have you tried disabling buffering   setvbuf fd  NULL   IONBF  0

User · Answer

fflush   only flushes the buffering added by the stdio fopen   layer  as managed by the FILE   object  The underlying file itself  as seen by the kernel  is not buffered at this level  This means that writes that bypass the FILE   layer  using fileno   and a raw write    are also not buffered in a way that fflush   would flush   As others have pointed out  try not mixing the two  If you need to use  raw  I O functions such as ioctl    then open   the file yourself directly  without using fopen lt    and friends from stdio

User · Answer

If you want to go the other way round  associate FILE  with existing file descriptor   use fdopen                                                                FDOPEN P   NAME         fdopen - associate a stream with a file descriptor  SYNOPSIS          include  lt stdio h gt          FILE  fdopen int fildes  const char  mode

[c] How can you flush a write using a file descriptor?

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