[GENERAL] Writing a C function to return the log file name

I am trying to write a function in C to return the log file name by given timestamp. I will use that later to make dynamic creation of a foreign table (file_fdw) to read the csv logs themselves. The thing is I do now want to hardcode neither the format, nor the directory in my extension.

I already looked into the adminpack extension, but the format is hardcoded to the default one there, so it does not serve my needs.

Here is what I currently have:

The thing is the function is returning random results, obtained by pg_strftime().