Use the STDSYS option in all mixed PL/I and C applications and all multithreading applications.
If you compile your program with the NOSTDSYS option, it might conflict with the use of the SYSIN DD name and the SYSPRINT DD name in C.
For example, if your program has the ATTACH statement, it starts the C environment directly. Starting the C environment causes the SYSIN and SYSPRINT streams to be opened and closed independent of your PL/I application programs. In same cases, SYSIN might fail to open or the SYSPRINT data set might be overwritten.