libbb/xrealloc_vector.c

   1 /* vi: set sw=4 ts=4: */
   2 /*
   3  * Utility routines.
   4  *
   5  * Copyright (C) 2008 Denys Vlasenko
   6  *
   7  * Licensed under GPLv2, see file LICENSE in this source tree.
   8  */
   9 #include "libbb.h"
  10
  11 /* Resize (grow) malloced vector.
  12  *
  13  *  #define magic packed two parameters into one:
  14  *  sizeof = sizeof_and_shift >> 8
  15  *  shift  = (sizeof_and_shift) & 0xff
  16  *
  17  * Lets say shift = 4. 1 << 4 == 0x10.
  18  * If idx == 0, 0x10, 0x20 etc, vector[] is resized to next higher
  19  * idx step, plus one: if idx == 0x20, vector[] is resized to 0x31,
  20  * thus last usable element is vector[0x30].
  21  *
  22  * In other words: after xrealloc_vector(v, 4, idx), with any idx,
  23  * it's ok to use at least v[idx] and v[idx+1].
  24  * v[idx+2] etc generally are not ok.
  25  *
  26  * New elements are zeroed out, but only if realloc was done
  27  * (not on every call). You can depend on v[idx] and v[idx+1] being
  28  * zeroed out if you use it like this:
  29  *  v = xrealloc_vector(v, 4, idx);
  30  *  v[idx].some_fields = ...; - the rest stays 0/NULL
  31  *  idx++;
  32  * If you do not advance idx like above, you should be more careful.
  33  * Next call to xrealloc_vector(v, 4, idx) may or may not zero out v[idx].
  34  */
  35 void* FAST_FUNC xrealloc_vector_helper(void *vector, unsigned sizeof_and_shift, int idx)
  36 {
  37         int mask = 1 << (uint8_t)sizeof_and_shift;
  38
  39         if (!(idx & (mask - 1))) {
  40                 sizeof_and_shift >>= 8; /* sizeof(vector[0]) */
  41                 vector = xrealloc(vector, sizeof_and_shift * (idx + mask + 1));
  42                 memset((char*)vector + (sizeof_and_shift * idx), 0, sizeof_and_shift * (mask + 1));
  43         }
  44         return vector;
  45 }