fsync does not write data to file

Question

I have two (POSIX) threads that write to a log file like this:

pthread_mutex_lock(&log_mutex);
fprintf(LOG, "something...\n");
fsync(fileno(LOG));
pthread_mutex_unlock(&log_mutex);

The file is opened in main() with fopen() with mode "a". While the process is running I can't see anything appearing in the file with cat or tail although after the process is terminated and the file is fclose()-ed, the lines are all there.

What am I doing wrong?

Answer 1

I guess you need to call fflush() to flush the changed to the file system.

See also difference between fsync() and fflush().

Answer 2

Since you are using FILE handle in C, you need to first flush the data from C/C++ buffers to kernel buffers by calling fflush(). fsync is not really required unless you also want to make sure the data reaches the underlying physical storage especially for durability concerns