pmdaOpenLog man page
pmdaOpenLog — redirect stderr to a logfile
void pmdaOpenLog(pmdaInterface * dispatch);
cc ... -lpcp_pmda -lpcp
pmdaOpenLog redirects stderr to the logfile specified in the dispatch structure, set by the previous call to pmdaDaemon(3) or pmdaGetOpt(3). The first line of the log file will detail the name of the calling process, the host the process is running on, and the current time. In addition, the log is appended with the exit time of the process by a routine registered with atexit(3).
The PMDA must be using PMDA_PROTOCOL_2 or later, as specified in the call to pmdaDSO(3) or pmdaDaemon(3).
Environment variables with the prefix PCP_ are used to parameterize the file and directory names used by PCP. On each installation, the file /etc/pcp.conf contains the local values for these variables. The $PCP_CONF variable may be used to specify an alternative configuration file, as described in pcp.conf(5). Values for these variables may be obtained programmatically using the pmGetConfig(3) function.
pmcd(1), atexit(2), PMAPI(3), PMDA(3), pmdaDaemon(3) and pmdaGetOpt(3).