Principally, there are two possibilities: Either the user provides the called subroutine, or TESSY does. If TESSY provides the called subroutine, TESSY generates a so-called “stub” function. TESSY provides two kinds of stub functions: stub functions, in which the user may insert C code; and stub functions, where TESSY checks the value of passed parameters and provides input to the function under test.
If the user provides the called subroutine, it can be done either in source form or in binary form. If the user provides the called subroutine, this is the first step away from module / unit testing towards integration testing.