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6.20.5.4 Foreign Structs

Finally, one last note on foreign values before moving on to actually calling foreign functions. Sometimes you need to deal with C structs, which requires interpreting each element of the struct according to the its type, offset, and alignment. Guile has some primitives to support this.

Scheme Procedure: sizeof type
C Function: scm_sizeof type

Return the size of type, in bytes.

type should be a valid C type, like int. Alternately type may be the symbol *, in which case the size of a pointer is returned. type may also be a list of types, in which case the size of a struct with ABI-conventional packing is returned.

Scheme Procedure: alignof type
C Function: scm_alignof type

Return the alignment of type, in bytes.

type should be a valid C type, like int. Alternately type may be the symbol *, in which case the alignment of a pointer is returned. type may also be a list of types, in which case the alignment of a struct with ABI-conventional packing is returned.

Guile also provides some convenience methods to pack and unpack foreign pointers wrapping C structs.

Scheme Procedure: make-c-struct types vals

Create a foreign pointer to a C struct containing vals with types types.

vals and types should be lists of the same length.

Scheme Procedure: parse-c-struct foreign types

Parse a foreign pointer to a C struct, returning a list of values.

types should be a list of C types.

For example, to create and parse the equivalent of a struct { int64_t a; uint8_t b; }:

(parse-c-struct (make-c-struct (list int64 uint8)
                               (list 300 43))
                (list int64 uint8))
⇒ (300 43)

As yet, Guile only has convenience routines to support conventionally-packed structs. But given the bytevector->foreign and foreign->bytevector routines, one can create and parse tightly packed structs and unions by hand. See the code for (system foreign) for details.


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