The action taken by the fflush() library function depends on the file mode. The fflush() function only flushes buffers in binary files with Variable or Undefined record format.
If you call one z/OS® XL C/C++ program from another z/OS XL C/C++ program by using the ANSI system() function, all open streams are flushed before control is passed to the callee, and again before control is returned to the caller. If you are running with POSIX(ON), a call to the POSIX system() function does not flush any streams to the system.