Modules/_ctypes/_ctypes.c (part 8)

Source:

cpython 3.14 @ ab2d84fe1023/Modules/_ctypes/_ctypes.c ↗

This annotation covers ctypes.Structure layout computation. See modules_ctypes7_detail for CDLL, CFUNCTYPE, byref, and pointer types.

Map

Lines	Symbol	Role
1-80	Structure_set_fields	Process _fields_ list and compute offsets
81-180	Field offset calculation	Alignment padding and _pack_ override
181-260	_anonymous_ handling	Flatten nested struct fields
261-360	Structure.__getattr__	Read a field value from the buffer
361-500	Structure.__setattr__	Write a field value into the buffer

Reading

Structure_set_fields

// CPython: Modules/_ctypes/_ctypes.c:3680 PyCStructType_set_fields
static int
PyCStructType_set_fields(PyObject *self, PyObject *value, void *Py_UNUSED(ignored))
{
    StgInfo *stginfo = PyStgInfo_GET(self);
    /* value is a list of (name, type) or (name, type, bitwidth) tuples */
    stginfo->fldnames = PyTuple_New(n);
    stginfo->fldtypes = PyTuple_New(n);
    for (int i = 0; i < n; i++) {
        /* Compute offset with alignment */
        Py_ssize_t size = type_stginfo->size;
        Py_ssize_t align = stginfo->align;
        /* align: pad to alignment boundary */
        offset = (total + align - 1) & ~(align - 1);
        total = offset + size;
    }
    stginfo->size = (total + max_align - 1) & ~(max_align - 1);
}

_fields_ is processed once at class creation time. Each field's offset is computed by rounding up total to the field type's alignment requirement. The struct's total size is padded to its own alignment.

_pack_ override

// CPython: Modules/_ctypes/_ctypes.c:3620 PyCStructType_new
/* If _pack_ is defined:
     align = min(type_align, _pack_)
   This matches MSVC's __pragma(pack(n)) and GCC's __attribute__((packed)). */
if (stginfo->pack && stginfo->pack < align)
    align = stginfo->pack;

_pack_ = 1 creates a packed struct with no padding, matching __attribute__((packed)) in C. This is needed for binary protocol structs that don't follow natural alignment.

Structure.__getattr__

// CPython: Modules/_ctypes/_ctypes.c:4080 Structure_getattro
static PyObject *
Structure_getattro(PyObject *self, PyObject *name)
{
    CDataObject *self_ = (CDataObject *)self;
    StgFieldInfo *fi = lookup_field(self_, name);
    if (fi == NULL) return PyObject_GenericGetAttr(self, name);
    /* Read bytes at offset from self->b_ptr */
    char *ptr = self_->b_ptr + fi->offset;
    return fi->proto->getfunc(ptr, fi->size);
}

Each field has a getfunc that converts raw bytes at the field's offset to a Python object. For c_int, getfunc reads 4 bytes and returns a Python int.

gopy notes

Structure_set_fields is module/ctypes.StructureSetFields in module/ctypes/structure.go. Field offsets are computed with Go's unsafe.Alignof-style arithmetic. Structure.__getattr__ reads from the buffer using field descriptors. _pack_ overrides the computed alignment.