Derived Types and struct - The GNU Fortran Compiler
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#### 7.1.2 Derived Types and struct

For compatibility of derived types with `struct`, one needs to use the `BIND(C)` attribute in the type declaration. For instance, the following type declaration

          USE ISO_C_BINDING
          TYPE, BIND(C) :: myType
            INTEGER(C_INT) :: i1, i2
            INTEGER(C_SIGNED_CHAR) :: i3
            REAL(C_DOUBLE) :: d1
            COMPLEX(C_FLOAT_COMPLEX) :: c1
            CHARACTER(KIND=C_CHAR) :: str(5)
          END TYPE

matches the following `struct` declaration in C

          struct {
            int i1, i2;
            /* Note: "char" might be signed or unsigned.  */
            signed char i3;
            double d1;
            float _Complex c1;
            char str[5];
          } myType;

Derived types with the C binding attribute shall not have the `sequence` attribute, type parameters, the `extends` attribute, nor type-bound procedures. Every component must be of interoperable type and kind and may not have the `pointer` or `allocatable` attribute. The names of the components are irrelevant for interoperability.

As there exist no direct Fortran equivalents, neither unions nor structs with bit field or variable-length array members are interoperable.