Linux – How to you flush a write using a file descriptor


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

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);


Best Solution

I think what you are looking for may be

int fsync(int fd);


int fdatasync(int fd);

fsync will flush the file from kernel buffer to the disk. fdatasync will also do except for the meta data.