[cups] Why a backend owned by root can't create file in /var/log/cups/ (Permission Denied)

[caiweiwenjs]

Dear all,
I install cups-pdf(come from http://www.cups-pdf.de/ ) in my computer, but it don't work.
I find the reason by printing debug information in a file.
The reason is permission denied: cups-pdf create log file in /var/log/cups/ and create directory in /var/spool/
This is my backend:
    cww at ubuntu:/usr/lib/cups/backend$ ll cups-pdf 
    -rwx------ 1 root root 138352 Dec 23 15:19 cups-pdf*
I modify source code cups-pdf.c which printing the result of getuid() and geteuid() in the begin of main(), the result both are 0.


Platform: Ubuntu 15.10 64bit
CUPS Version: CUPS 2.1.0
CUPS-PDF version: CUPS-PDF 3.0beta2


Thank you very much!
Sorry for my poor English.


Weiwen Cai