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

9 files changed, 30 insertions, 20 deletions

diff --git a/archival/dpkg.c b/archival/dpkg.c
index 9ea308703..54e963233 100644
--- a/archival/dpkg.c
+++ b/archival/dpkg.c
@@ -1169,10 +1169,9 @@ static char **create_list(const char *filename)
 	file_list = NULL;
 	count = 0;
 	while ((line = xmalloc_fgetline(list_stream)) != NULL) {
-//TODO: zeroing xrealloc_vector?
 		file_list = xrealloc_vector(file_list, 2, count);
 		file_list[count++] = line;
-		file_list[count] = NULL;
+		/*file_list[count] = NULL; - xrealloc_vector did it */
 	}
 	fclose(list_stream);
 
diff --git a/debianutils/run_parts.c b/debianutils/run_parts.c
index cb5f48fdd..a58fab3a5 100644
--- a/debianutils/run_parts.c
+++ b/debianutils/run_parts.c
@@ -92,7 +92,7 @@ static int FAST_FUNC act(const char *file, struct stat *statbuf, void *args UNUS
 
 	names = xrealloc_vector(names, 4, cur);
 	names[cur++] = xstrdup(file);
-	names[cur] = NULL;
+	/*names[cur] = NULL; - xrealloc_vector did it */
 
 	return TRUE;
 }
diff --git a/include/libbb.h b/include/libbb.h
index 2dcdeacb1..f7a68492c 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -570,6 +570,10 @@ void *malloc_or_warn(size_t size) FAST_FUNC;
 void *xmalloc(size_t size) FAST_FUNC;
 void *xzalloc(size_t size) FAST_FUNC;
 void *xrealloc(void *old, size_t size) FAST_FUNC;
+/* After xrealloc_vector(v, 4, idx) it's ok to use
+ * at least v[idx] and v[idx+1], for all idx values.
+ * shift specifies how many new elements are added (1: 2, 2: 4... 8: 256...)
+ * when all elements are used up. New elements are zeroed out. */
 #define xrealloc_vector(vector, shift, idx) \
 	xrealloc_vector_helper((vector), (sizeof((vector)[0]) << 8) + (shift), (idx))
 void* xrealloc_vector_helper(void *vector, unsigned sizeof_and_shift, int idx) FAST_FUNC;
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;
 }
diff --git a/miscutils/man.c b/miscutils/man.c
index edeffc776..f499fef62 100644
--- a/miscutils/man.c
+++ b/miscutils/man.c
@@ -112,7 +112,7 @@ int man_main(int argc UNUSED_PARAM, char **argv)
 			man_path_list[count_mp] = xstrdup(token[1]);
 			count_mp++;
 			/* man_path_list is NULL terminated */
-			man_path_list[count_mp] = NULL;
+			/*man_path_list[count_mp] = NULL; - xrealloc_vector did it */
 		}
 		if (strcmp("MANSECT", token[0]) == 0) {
 			free(sec_list);
diff --git a/modutils/modprobe-small.c b/modutils/modprobe-small.c
index 63be9aadb..4aa2b5862 100644
--- a/modutils/modprobe-small.c
+++ b/modutils/modprobe-small.c
@@ -270,10 +270,9 @@ static FAST_FUNC int fileAction(const char *pathname,
 
 	cur = module_count++;
 	modinfo = xrealloc_vector(modinfo, 12, cur);
-//TODO: use zeroing version of xrealloc_vector?
 	modinfo[cur].pathname = xstrdup(pathname);
-	modinfo[cur].aliases = NULL;
-	modinfo[cur+1].pathname = NULL;
+	/*modinfo[cur].aliases = NULL; - xrealloc_vector did it */
+	/*modinfo[cur+1].pathname = NULL;*/
 
 	if (!pathname_matches_modname(fname, modname_to_match)) {
 		dbg1_error_msg("'%s' module name doesn't match", pathname);
@@ -330,8 +329,7 @@ static int load_dep_bb(void)
 		space = strchrnul(line, ' ');
 		cur = module_count++;
 		modinfo = xrealloc_vector(modinfo, 12, cur);
-//TODO: use zeroing version of xrealloc_vector?
-		modinfo[cur+1].pathname = NULL;
+		/*modinfo[cur+1].pathname = NULL; - xrealloc_vector did it */
 		modinfo[cur].pathname = line; /* we take ownership of malloced block here */
 		if (*space)
 			*space++ = '\0';
diff --git a/procps/top.c b/procps/top.c
index 1a6b8abb2..1f1415f83 100644
--- a/procps/top.c
+++ b/procps/top.c
@@ -814,7 +814,7 @@ int top_main(int argc UNUSED_PARAM, char **argv)
 			int n;
 			if (scan_mask == TOP_MASK) {
 				n = ntop;
-				top = xrealloc_vector(top, 2, ntop++);
+				top = xrealloc_vector(top, 6, ntop++);
 				top[n].pid = p->pid;
 				top[n].ppid = p->ppid;
 				top[n].vsz = p->vsz;
@@ -830,7 +830,7 @@ int top_main(int argc UNUSED_PARAM, char **argv)
 					continue; /* kernel threads are ignored */
 				n = ntop;
 				/* No bug here - top and topmem are the same */
-				top = xrealloc_vector(topmem, 2, ntop++);
+				top = xrealloc_vector(topmem, 6, ntop++);
 				strcpy(topmem[n].comm, p->comm);
 				topmem[n].pid      = p->pid;
 				topmem[n].vsz      = p->mapped_rw + p->mapped_ro;
diff --git a/util-linux/getopt.c b/util-linux/getopt.c
index 25aa4069a..402630385 100644
--- a/util-linux/getopt.c
+++ b/util-linux/getopt.c
@@ -238,14 +238,13 @@ static struct option *add_long_options(struct option *long_options, char *option
 				if (tlen == 0)
 					bb_error_msg_and_die("empty long option specified");
 			}
-//TODO: zeroing version of xrealloc_vector!
 			long_options = xrealloc_vector(long_options, 4, long_nr);
 			long_options[long_nr].has_arg = arg_opt;
-			long_options[long_nr].flag = NULL;
+			/*long_options[long_nr].flag = NULL; - xrealloc_vector did it */
 			long_options[long_nr].val = LONG_OPT;
 			long_options[long_nr].name = xstrdup(tokptr);
 			long_nr++;
-			memset(&long_options[long_nr], 0, sizeof(long_options[0]));
+			/*memset(&long_options[long_nr], 0, sizeof(long_options[0])); - xrealloc_vector did it */
 		}
 		tokptr = strtok(NULL, ", \t\n");
 	}