Question

I have a C program which has multiple worker threads. There is a main thread which periodically (every 0.2s) does some basic checks (i.e. has a thread finished, has a signal been received, etc). At each check, I would like to write to a log file any data that any of the threads may have in their log buffer to a single log file.

My initial idea was to simply open the log file, write the data from all the threads and then close it again. I am worried that this might be too much of an overhead seeing as these checks occur every 0.2s.

So my question is – is this scenario inefficient?

If so, can anyone suggest a better solution?

I thought of leaving the file descriptor open and just writing new data on every check, but then there is the problem if somehow the physical file gets deleted, the program would never know (without rechecking, and in this case we might as well just open the file again) and logging data would be lost.

(This program is designed to run for very long periods of time, so the fact that log file will be deleted at some point is basically guaranteed due to log rotation.)

Answer 1

I thought of leaving the file descriptor open and just writing new data on every check, but then there is the problem if somehow the physical file gets deleted, the program would never know (without rechecking, and in this case we might as well just open the file again) and logging data would be lost.

The standard solution on UNIX is to add a signal handler for SIGHUP which closes and re-opens the log file. Many UNIX daemons do this for precisely this purpose, to support log rotation. Call kill -HUP <pid> in your log rotation script and you’re good to go.

(Some programs will also treat SIGHUP as a cue to re-read their configuration files, so you can make configuration changes on the fly without having to restart processes.)