The C function must be considered as a procedure i.e. its type must be void or the returned value must not be used.
The arguments of the C function must be considered as Fortran arguments i.e. they must be only pointers.
Moreover, the name of the C function must be recognized by Fortran. For that, the include file machine.h located in the directory <Scilab directory>/routines should be included in C functions and the macro C2F should be used.