Associating the I/O streams

If, when linking the program, you specified the POSIX linkage option and POSIX is active, the standard I/O streams stdin, stdout and stderr are opened via POSIX.
In batch jobs, procedures or if the PROGRAM_ENVIRONMENT environment variable is not set to SHELL, the standard I/O streams are associated via POSIX with the BS2000 system files (SYSDTA, SYSOUT), otherwise with the terminal.

Without POSIX, the standard I/O streams stdin, stdout and stderr are directly associated with the BS2000 system files (SYSDTA, SYSOUT).