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

1 files changed, 4 insertions, 0 deletions

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;