bpo-30242: resolve some undefined behaviours in struct (#1418) (#1588)

Modules/_struct.c

@@ -506,7 +506,7 @@ np_ubyte(char *p, PyObject *v, const formatdef *f)
                         "ubyte format requires 0 <= number <= 255");
         return -1;
     }
-    *p = (char)x;
+    *(unsigned char *)p = (unsigned char)x;
     return 0;
 }
 
@@ -814,6 +814,7 @@ bp_int(char *p, PyObject *v, const formatdef *f)
 {
     long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_long(v, &x) < 0)
         return -1;
     i = f->size;
@@ -826,7 +827,7 @@ bp_int(char *p, PyObject *v, const formatdef *f)
 #endif
     }
     do {
-        p[--i] = (char)x;
+        q[--i] = (unsigned char)(x & 0xffL);
         x >>= 8;
     } while (i > 0);
     return 0;
@@ -837,6 +838,7 @@ bp_uint(char *p, PyObject *v, const formatdef *f)
 {
     unsigned long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_ulong(v, &x) < 0)
         return -1;
     i = f->size;
@@ -847,7 +849,7 @@ bp_uint(char *p, PyObject *v, const formatdef *f)
             return _range_error(f, 1);
     }
     do {
-        p[--i] = (char)x;
+        q[--i] = (unsigned char)(x & 0xffUL);
         x >>= 8;
     } while (i > 0);
     return 0;
@@ -1034,6 +1036,7 @@ lp_int(char *p, PyObject *v, const formatdef *f)
 {
     long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_long(v, &x) < 0)
         return -1;
     i = f->size;
@@ -1046,7 +1049,7 @@ lp_int(char *p, PyObject *v, const formatdef *f)
 #endif
     }
     do {
-        *p++ = (char)x;
+        *q++ = (unsigned char)(x & 0xffL);
         x >>= 8;
     } while (--i > 0);
     return 0;
@@ -1057,6 +1060,7 @@ lp_uint(char *p, PyObject *v, const formatdef *f)
 {
     unsigned long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_ulong(v, &x) < 0)
         return -1;
     i = f->size;
@@ -1067,7 +1071,7 @@ lp_uint(char *p, PyObject *v, const formatdef *f)
             return _range_error(f, 1);
     }
     do {
-        *p++ = (char)x;
+        *q++ = (unsigned char)(x & 0xffUL);
         x >>= 8;
     } while (--i > 0);
     return 0;