Warning: This is the manual of the legacy Guile 2.2 series. You may want to read the manual of the current stable series instead.

6.21.5.4 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->pointer and pointer->bytevector routines, one can create and parse tightly packed structs and unions by hand. See the code for (system foreign) for details.