See discussion at https://fortran-lang.discourse.group/t/best-practices-for-passing-c-strings/104/12

fc_string_funcs.f90

subroutine C_F_STRPOINTER (STRARRAY, FSTRPTR, MAXLEN)
  use, intrinsic :: ISO_C_BINDING
  implicit none
  character, dimension(*), target, intent(in) :: STRARRAY
  character(:), pointer, intent(out) :: FSTRPTR
  integer, intent(in), optional :: MAXLEN

  integer :: curlen

  curlen = 0
  do
    curlen = curlen +1
    if (PRESENT(MAXLEN)) THEN
      if (curlen > MAXLEN) exit
    end if
    if (STRARRAY(CURLEN) == CHAR(0)) exit
    end do

  call doassign(C_LOC(STRARRAY), FSTRPTR, curlen-1)

  contains
  subroutine doassign(CSTRPTR, FSTRPTR, STRLEN)
  type(C_PTR), intent(in) :: CSTRPTR
  character(:), pointer, intent(out) :: FSTRPTR
  integer, intent(in) :: STRLEN

  character(STRLEN), pointer :: p

  call C_F_POINTER(CSTRPTR, p)
  FSTRPTR => p
  return
  end subroutine doassign
end subroutine C_F_STRPOINTER


function f_c_string(string,trim)
  use, intrinsic :: iso_c_binding, only: c_char,c_null_char
  character(len=*), intent(in) :: string
  logical, intent(in), optional :: trim

  character(kind=c_char,len=:), allocatable :: f_c_string
  logical :: trim_

  trim_ = .true.
  if (present(trim)) trim_ = trim

  block
    intrinsic trim
    if (trim_) then
      f_c_string = trim(string)//c_null_char
    else
      f_c_string = string//c_null_char
    end if
  end block
end function