Jamie Jamie - 4 months ago 29
C Question

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

You have two choices:

  1. Use fileno() to obtain the file descriptor associated with the stdio stream pointer

  2. Don't use <stdio.h> 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);
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