You must evaluate existing C applications for thread safety before calling them in a multithreaded job. If your C program is not threadsafe, techniques are available for calling programs that are not threadsafe from multithreaded jobs.
Not all C library functions are threadsafe. Before
calling your existing C applications in a multithread-capable job, refer to
the WebSphere® Development
Studio: ILE C/C++ Programmer's Guide, SC09-2712 PDF 
and WebSphere Development Studio: C/C++ Language Reference,
SC09-4815 PDF to determine whether all of your functions are threadsafe.
- Re-creation of Integrated Language Environment® (ILE)
C applications:
Compile and bind all existing ILE C applications with TGTRLS(*CURRENT) before you call them in a multithreaded job.
- Elimination of *NEW activation groups:You should not use *NEW activation groups in multithreaded applications.Note: ACTGRP(*NEW) is the default on CRTPGM and CRTBNDC commands.
- Calling ILE C *PGM objects:
If you need to call an ILE C *PGM object that has an activation group of *NEW, either start another process to run the second program or create the second program specifying a named activation group.