Issue #20160: Handled passing of large structs to callbacks correctly.

2 files changed, 30 insertions, 2 deletions

diff --git a/Modules/_ctypes/_ctypes_test.c b/Modules/_ctypes/_ctypes_test.c
index 57628b8993..f295c6fc4c 100644
--- a/Modules/_ctypes/_ctypes_test.c
+++ b/Modules/_ctypes/_ctypes_test.c
@@ -34,6 +34,24 @@ _testfunc_cbk_reg_double(double a, double b, double c, double d, double e,
     return func(a*a, b*b, c*c, d*d, e*e);
 }
 
+/*
+ * This structure should be the same as in test_callbacks.py and the
+ * method test_callback_large_struct. See issues 17310 and 20160: the
+ * structure must be larger than 8 bytes long.
+ */
+
+typedef struct {
+    unsigned long first;
+    unsigned long second;
+    unsigned long third;
+} Test;
+
+EXPORT(void)
+_testfunc_cbk_large_struct(Test in, void (*func)(Test))
+{
+    func(in);
+}
+
 EXPORT(void)testfunc_array(int values[4])
 {
     printf("testfunc_array %d %d %d %d\n",
diff --git a/Modules/_ctypes/libffi_msvc/ffi.c b/Modules/_ctypes/libffi_msvc/ffi.c
index ab6b2209d6..515d802fd8 100644
--- a/Modules/_ctypes/libffi_msvc/ffi.c
+++ b/Modules/_ctypes/libffi_msvc/ffi.c
@@ -359,7 +359,7 @@ ffi_prep_incoming_args_SYSV(char *stack, void **rvalue,
 
   if ( cif->rtype->type == FFI_TYPE_STRUCT ) {
     *rvalue = *(void **) argp;
-    argp += 4;
+    argp += sizeof(void *);
   }
 
   p_argv = avalue;
@@ -370,13 +370,23 @@ ffi_prep_incoming_args_SYSV(char *stack, void **rvalue,
 
       /* Align if necessary */
       if ((sizeof(char *) - 1) & (size_t) argp) {
-	argp = (char *) ALIGN(argp, sizeof(char*));
+        argp = (char *) ALIGN(argp, sizeof(char*));
       }
 
       z = (*p_arg)->size;
 
       /* because we're little endian, this is what it turns into.   */
 
+#ifdef _WIN64
+      if (z > 8) {
+        /* On Win64, if a single argument takes more than 8 bytes,
+         * then it is always passed by reference.
+         */
+        *p_argv = *((void**) argp);
+        z = 8;
+      }
+      else
+#endif
       *p_argv = (void*) argp;
 
       p_argv++;