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?

Maybe [you want to use fflush()](http://stackoverflow.com/questions/2340610/difference-between-fflush-and-fsync)? — Manuel Faux, Mar 17 '13 at 09:40

score 8 · Accepted Answer · edited May 23 '17 at 12:33

8

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

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

edited May 23 '17 at 12:33

Community

1
1

answered Mar 17 '13 at 09:42

Manuel Faux

2,317
5
24
35

score 1 · Answer 2 · answered Sep 05 '13 at 21:44

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

fsync does not write data to file

2 Answers2