A CICS transient data queue is a pathway to a single predefined destination. The destination can be a ddname, another transient data queue, a VSAM file, a terminal, or another CICS environment. The CICS system administrator defines the queues that are active during execution of CICS. All users who direct data to a given queue will be placing data in the same location, in order of occurrence.
You cannot use fopen() or freopen() to specify this kind of I/O. It is the category selected automatically when you call any ANSI functions that reference stdout and stderr under CICS. If you reference either of these in a C or C++ program under CICS, z/OS® XL C/C++ attempts to open the CESO (stdout) or CESE (stderr) queue. If you want to write to any other queue, you should use the CICS-provided interface.