FILE *pmOpenLog(const char * progname , const char * logname , FILE * oldstream , int * status );
As a special case, if logname is " - " then no removal or reopening is performed and the function simply sets status to 1 and returns oldstream . This is useful when the caller wants diagnostics on oldstream stream (normally stderr ) rather than a file, e.g. pmlogger -l- or pmcd -f -l- . Logging to stderr is also useful for PMDAs in a containerized environment where it is beneficial for all PMDA logs to be written to pmcd 's stderr stream (and thus to a single destination), whether that is a file such as pmcd.log or the original stream inherited from the shell.
On return, the function value is the standard I/O stream, possibly replacing oldstream . In the event of an error, the return value will be oldstream unchanged and status will be 0 .
and a standard preamble is written to
Log for progname on hostname started date and time
and an atexit (3) handler is installed to write the postscript message to logname
Log finished date and time
when the processes exits.