On many systems the runtime libraries for a given language has to be initialized --- a task which is then performed by the main program.
With a Fortran main program calling C functions directly or indirectly the following points have to be respected.
C/370
The C/370 runtime library is automatically initialized when the first
C routine is called.
The system resources allocated there are released again as soon as the
Fortran routine, from which the C routine triggering the
initialization was called, returns.
Calling another C routine after that does not cause a
re-initialization and the program usually crashes because the
malloc heap has become invalid.
This effect can be safely avoided by calling the first C routine immediately
in the Fortran main program.
KERNLIB contains a dummy routine INITC for this purpose.
When executing a module containing C code the C runtime libraries have
to be included in the GLOBAL library lists.
The CERNLIB command is the most convenient call to this.
Note that the C compiler itself is a C program and will abort with an
unintelligible message if the libraries are not defined.
VMS
A Fortran program should call VAXC$CRTL_INIT (either from
Fortran or from C) before calling any functions from the C runtime
library.
There can be strange effects if this rule is not obeyed.
Our experience with a C main program calling Fortran routines is
limited.
One known restriction is that the GETARG routine for retrieving
the command line arguments in Fortran cannot be used.
Calling this routine usually results in unresolved externals called
f77_argc or similar.