Modules/_ctypes/callproc.c

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

Modules/_ctypes/callproc.c is the core of the ctypes call machinery. When a ctypes function object is called, it assembles a ffi_cif (call interface) from the argument types, converts each Python argument to its C representation using SETFUNC, calls the foreign function with GIL released via ffi_call, then converts the return value back to a Python object using GETFUNC.

Map

Lines	Symbol	Role
1-80	includes, PyCArgObject	Argument holder with type tag and union value
81-250	ConvParam	Python-to-C argument marshaling for each ctypes type
251-500	_CallProc	Main call assembler: build args array, call ffi_call
501-700	call_function	Python entry point; extracts errcheck callback
701-900	_ctypes_callproc	Low-level call with GIL management
901-1100	SetResult, GetResult	Return value conversion
1101-1600	error handling, GetLastError/errno	Per-thread errno/HRESULT capture

Reading

ConvParam: Python-to-C argument conversion

ConvParam converts a Python object to a PyCArgObject holding a C-typed value. It checks for _as_parameter_ protocol, then dispatches on the type's SETFUNC.

// CPython: Modules/_ctypes/callproc.c:145 ConvParam
static int
ConvParam(PyObject *obj, Py_ssize_t index, struct argument *pa)
{
    StgInfo *info;
    if (PyObject_TypeCheck(obj, &PyCArg_Type)) {
        PyCArgObject *carg = (PyCArgObject *)obj;
        *pa = carg->value;
        return 0;
    }
    if (CDataObject_Check(obj)) {
        ...
        pa->ffi_type = info->ffi_type_pointer;
        pa->value.p = ((CDataObject *)obj)->b_ptr;
        return 0;
    }
    ...

_CallProc: ffi_cif assembly and dispatch

_CallProc builds the ffi_type * array for arguments, calls ffi_prep_cif, then releases the GIL and calls ffi_call.

// CPython: Modules/_ctypes/callproc.c:330 _CallProc
static PyObject *
_CallProc(PPROC pProc, PyObject *argtuple, ...)
{
    ffi_cif cif;
    ffi_type **atypes;
    ...
    result = ffi_prep_cif(&cif, cc, argcount, rtype, atypes);
    if (result != FFI_OK) { ... }
    Py_BEGIN_ALLOW_THREADS
    ffi_call(&cif, pProc, resmem, avalues);
    Py_END_ALLOW_THREADS
    ...

Per-thread errno capture

After each foreign call, ctypes captures the C errno (and GetLastError on Windows) into per-thread storage so ctypes.get_errno() can return it safely from any thread.

// CPython: Modules/_ctypes/callproc.c:460 _ctypes_save_errno
static void
_ctypes_save_errno(void)
{
    errno_t *ts_errno = &_PyRuntime.errno_state.eno;
    *ts_errno = errno;
#ifdef MS_WIN32
    ...
#endif
}

gopy notes

Not yet ported. Go's cgo provides direct C function calls without an intermediate libffi layer. For a ctypes port at module/_ctypes/, the _CallProc machinery would need to either use cgo or embed libffi. The PyCArgObject protocol maps to a Go interface with a CValue() unsafe.Pointer method.

CPython 3.14 changes

3.14 added support for ffi_type_complex_float and ffi_type_complex_double in the argument type registry, enabling ctypes functions to accept C _Complex float arguments. The _ctypes_save_errno mechanism was unified with the new errno thread-state field.