CODEWITHSUNDEEP
cfsync
Michael Seiwald
Michael Seiwald

Reputation: 237

fsync does not write data to file

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?

Upvotes: 4

Views: 1736

Answers (2)

jayadev
jayadev

Reputation: 3711

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

Upvotes: 1

Manuel Faux
Manuel Faux

Reputation: 2457

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

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

Upvotes: 9

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