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");
         }