• the bind(C) interface is a vital capability that couples to external programs written in C and C++.
• This project does not use logical(C_BOOL) at bind(C) interfaces due to possibility of bugs with distinct length integers (1 byte) vs. logical (4 bytes) in Fortran. Instead, we use integer(C_INT) and convert to and from logical in Fortran.
• It's perfectly fine to use Fortran logical inside this program, and even inside procedures that are bind(C), as long as the interface to C uses integer(C_INT) and the conversion is handled correctly in Fortran.
• to convert from integer(C_INT) to logical, we check if the integer is not equal to zero. This is a common convention in C where zero represents false and any non-zero value represents true. For example:

logical :: flag_fortran
integer(C_INT) :: flag_c
flag_fortran = flag_c /= 0

• to convert from logical to integer(C_INT), we can use a simple conditional expression that returns 1 for true and 0 for false. This ensures that the values are correctly interpreted when passed to C functions. For example:

logical :: flag_fortran
integer(C_INT) :: flag_c
flag_c = merge(1, 0, flag_fortran)

• examples of these interfaces are in src/libgemini_c.f90 get_config_vars_C() subroutine, where we convert the flagneuBG from integer(C_INT) to logical for use in Fortran, and then convert it back to integer(C_INT) before returning to C.

For reference, there were only a couple places that used logical(C_BOOL) in the original code.

• src/libgemini.f90:type(c_params) has members of type integer(C_INT) that represent boolean flags: fortran_nml, fortran_cli, debug, dryrun.
• src/libgemini_c.f90:get_config_vars_C() has a parameter integer(C_INT) :: flagneuBG used to receive the value from C.