How can 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
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?
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.