The GNU Fortran Compiler: Interoperable Global Variables

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#### 6.1.3 Interoperable Global Variables

Variables can be made accessible from C using the C binding attribute, optionally together with specifying a binding name. Those variables have to be declared in the declaration part of a `MODULE`, be of interoperable type, and have neither the `pointer` nor the `allocatable` attribute.

      MODULE m
        USE myType_module
        USE ISO_C_BINDING
        integer(C_INT), bind(C, name="_MyProject_flags") :: global_flag
        type(myType), bind(C) :: tp
      END MODULE

Here, `_MyProject_flags` is the case-sensitive name of the variable as seen from C programs while `global_flag` is the case-insensitive name as seen from Fortran. If no binding name is specified, as for tp, the C binding name is the (lowercase) Fortran binding name. If a binding name is specified, only a single variable may be after the double colon. Note of warning: You cannot use a global variable to access errno of the C library as the C standard allows it to be a macro. Use the `IERRNO` intrinsic (GNU extension) instead.