Design Approach form C to C++ Interaction

We have a RTOS library code in C which has a for loop something like below:

int exec{}
{
    for(;;)
    {
      ...................
       getbuffer(&req) || processbuffer(&req)
       {
          error = -1;
          return error;
       }
     }
}

int getbuffer(buf *req)
{
  ......
  fn1();
  .......
}

int fn1()
{
    .................
}

In fn1 I am implementing a new code that calls a callback but cannot wait for it's execution- the callback will be called by another thread.

This is the issue - function getbuffer resumes flow resumes even before callback is called. The expected scenario is that callback should be called first followed by further execution of getbuffer flow?

How can I achieve the same?