The stdin, stdout, and stderr I/O destinations (SYS$INPUT, SYS$OUTPUT, and SYS$ERROR) are all controlled by PMDF and will vary depending upon the context under which a channel program has been invoked. As such, programs which will operate as PMDF channels should use the PMDF_log routine described in Chapter 1 to write information to their log file. Such programs should never write output directly to stdout or stderr or other generic I/O destinations (e.g., Pascal's "output" or FORTRAN's default output logical unit). There's no telling where such output might go: it might go to the job controller's log file, it might even go down a network pipe to a remote client or server.
Note that the channel log file is a different file than the PMDF log file; the PMDF_log and PMDF_close_log_file are unrelated routines.