py/modstruct: Support pad bytes in struct format.

This adds support for the x format code in struct.pack and struct.unpack.

The primary use case for this is ignoring bytes while unpacking.  When
interfacing with existing systems, it may often happen that you either have
fields in a struct that aren't properly specified or you simply don't care
about them.  Being able to easily skip them is useful.

Signed-off-by: Daniël van de Giessen <daniel@dvdgiessen.nl>

py/modstruct.c

@@ -92,7 +92,9 @@ STATIC size_t calc_size_items(const char *fmt, size_t *total_sz) {
             cnt = get_fmt_num(&fmt);
         }
 
-        if (*fmt == 's') {
+        if (*fmt == 'x') {
+            size += cnt;
+        } else if (*fmt == 's') {
             total_cnt += 1;
             size += cnt;
         } else {
@@ -159,7 +161,9 @@ STATIC mp_obj_t struct_unpack_from(size_t n_args, const mp_obj_t *args) {
             cnt = get_fmt_num(&fmt);
         }
         mp_obj_t item;
-        if (*fmt == 's') {
+        if (*fmt == 'x') {
+            p += cnt;
+        } else if (*fmt == 's') {
             item = mp_obj_new_bytes(p, cnt);
             p += cnt;
             res->items[i++] = item;
@@ -192,7 +196,10 @@ STATIC void struct_pack_into_internal(mp_obj_t fmt_in, byte *p, size_t n_args, c
             cnt = get_fmt_num(&fmt);
         }
 
-        if (*fmt == 's') {
+        if (*fmt == 'x') {
+            memset(p, 0, cnt);
+            p += cnt;
+        } else if (*fmt == 's') {
             mp_buffer_info_t bufinfo;
             mp_get_buffer_raise(args[i++], &bufinfo, MP_BUFFER_READ);
             mp_uint_t to_copy = cnt;

tests/basics/struct1.py

@@ -20,6 +20,8 @@ print(struct.pack("<h", 1))
 print(struct.pack(">h", 1))
 print(struct.pack("<b", 1))
 print(struct.pack(">b", 1))
+print(struct.pack("<x"))
+print(struct.pack(">x"))
 
 print(struct.pack("<bI", -128, 256))
 print(struct.pack(">bI", -128, 256))
@@ -29,6 +31,13 @@ print(struct.calcsize("97sI"))
 print(struct.unpack("<6sH", b"foo\0\0\0\x12\x34"))
 print(struct.pack("<6sH", b"foo", 10000))
 
+print(struct.calcsize("7xx"))
+print(struct.pack("7xx"))
+
+print(struct.calcsize(">bxI3xH"))
+print(struct.pack(">bxI3xH", 1, 2, 3))
+print(struct.unpack(">bxI3xH", b"\x01\0\0\0\0\x02\0\0\0\0\x03"))
+
 s = struct.pack("BHBI", 10, 100, 200, 300)
 v = struct.unpack("BHBI", s)
 print(v == (10, 100, 200, 300))