libbb: make xrealloc_vector zero out the realloc'ed tail

function                                             old     new   delta
xrealloc_vector_helper                                51      76     +25
man_main                                             712     705      -7
act                                                  250     234     -16
create_list                                           91      70     -21
getopt_main                                          695     664     -31
load_dep_bb                                          281     248     -33
fileAction                                           744     709     -35
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/6 up/down: 25/-143)          Total: -118 bytes

2 files changed, 16 insertions, 6 deletions

diff --git a/libbb/procps.c b/libbb/procps.c
index ba3d25050..fd19621db 100644
--- a/libbb/procps.c
+++ b/libbb/procps.c
@@ -45,8 +45,8 @@ static int get_cached(cache_t *cp, unsigned id)
 	for (i = 0; i < cp->size; i++)
 		if (cp->cache[i].id == id)
 			return i;
-	i = cp->size;
-	cp->cache = xrealloc_vector(cp->cache, 2, cp->size++);
+	i = cp->size++;
+	cp->cache = xrealloc_vector(cp->cache, 2, i);
 	cp->cache[i++].id = id;
 	return -i;
 }
@@ -59,8 +59,8 @@ static char* get_cached(cache_t *cp, unsigned id, ug_func* fp)
 	for (i = 0; i < cp->size; i++)
 		if (cp->cache[i].id == id)
 			return cp->cache[i].name;
-	i = cp->size;
-	cp->cache = xrealloc_vector(cp->cache, 2, cp->size++);
+	i = cp->size++;
+	cp->cache = xrealloc_vector(cp->cache, 2, i);
 	cp->cache[i].id = id;
 	/* Never fails. Generates numeric string if name isn't found */
 	fp(cp->cache[i].name, sizeof(cp->cache[i].name), id);
diff --git a/libbb/xrealloc_vector.c b/libbb/xrealloc_vector.c
index ccc961b7b..342ae536e 100644
--- a/libbb/xrealloc_vector.c
+++ b/libbb/xrealloc_vector.c
@@ -14,7 +14,6 @@
  *  #define magic packed two parameters into one:
  *  sizeof = sizeof_and_shift >> 8
  *  shift  = (sizeof_and_shift) & 0xff
- *  (TODO: encode "I want it zeroed" in lowest bit?)
  *
  * Lets say shift = 4. 1 << 4 == 0x10.
  * If idx == 0, 0x10, 0x20 etc, vector[] is resized to next higher
@@ -23,14 +22,25 @@
  *
  * In other words: after xrealloc_vector(v, 4, idx) it's ok to use
  * at least v[idx] and v[idx+1], for all idx values.
+ *
+ * New elements are zeroed out, but only if realloc was done
+ * (not on every call). You can depend on v[idx] and v[idx+1] being
+ * zeroed out if you use it like this:
+ *  v = xrealloc_vector(v, 4, idx);
+ *  v[idx].some_fields = ...; - the rest stays 0/NULL
+ *  idx++;
+ * If you do not advance idx like above, you should be more careful.
+ * Next call to xrealloc_vector(v, 4, idx) may or may not zero out v[idx].
  */
 void* FAST_FUNC xrealloc_vector_helper(void *vector, unsigned sizeof_and_shift, int idx)
 {
 	int mask = 1 << (uint8_t)sizeof_and_shift;
 
 	if (!(idx & (mask - 1))) {
-		sizeof_and_shift >>= 8;
+		sizeof_and_shift >>= 8; /* sizeof(vector[0]) */
 		vector = xrealloc(vector, sizeof_and_shift * (idx + mask + 1));
+		vector += idx;
+		memset(vector, 0, sizeof_and_shift * (mask + 1));
 	}
 	return vector;
 }