gl/xsize.h

   1 /* xsize.h -- Checked size_t computations.
   2
   3    Copyright (C) 2003, 2008-2012 Free Software Foundation, Inc.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3, or (at your option)
   8    any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program; if not, see <http://www.gnu.org/licenses/>.  */
  17
  18 #ifndef _XSIZE_H
  19 #define _XSIZE_H
  20
  21 /* Get size_t.  */
  22 #include <stddef.h>
  23
  24 /* Get SIZE_MAX.  */
  25 #include <limits.h>
  26 #if HAVE_STDINT_H
  27 # include <stdint.h>
  28 #endif
  29
  30 _GL_INLINE_HEADER_BEGIN
  31 #ifndef XSIZE_INLINE
  32 # define XSIZE_INLINE _GL_INLINE
  33 #endif
  34
  35 /* The size of memory objects is often computed through expressions of
  36    type size_t. Example:
  37       void* p = malloc (header_size + n * element_size).
  38    These computations can lead to overflow.  When this happens, malloc()
  39    returns a piece of memory that is way too small, and the program then
  40    crashes while attempting to fill the memory.
  41    To avoid this, the functions and macros in this file check for overflow.
  42    The convention is that SIZE_MAX represents overflow.
  43    malloc (SIZE_MAX) is not guaranteed to fail -- think of a malloc
  44    implementation that uses mmap --, it's recommended to use size_overflow_p()
  45    or size_in_bounds_p() before invoking malloc().
  46    The example thus becomes:
  47       size_t size = xsum (header_size, xtimes (n, element_size));
  48       void *p = (size_in_bounds_p (size) ? malloc (size) : NULL);
  49 */
  50
  51 /* Convert an arbitrary value >= 0 to type size_t.  */
  52 #define xcast_size_t(N) \
  53   ((N) <= SIZE_MAX ? (size_t) (N) : SIZE_MAX)
  54
  55 /* Sum of two sizes, with overflow check.  */
  56 XSIZE_INLINE size_t
  57 #if __GNUC__ >= 3
  58 __attribute__ ((__pure__))
  59 #endif
  60 xsum (size_t size1, size_t size2)
  61 {
  62   size_t sum = size1 + size2;
  63   return (sum >= size1 ? sum : SIZE_MAX);
  64 }
  65
  66 /* Sum of three sizes, with overflow check.  */
  67 XSIZE_INLINE size_t
  68 #if __GNUC__ >= 3
  69 __attribute__ ((__pure__))
  70 #endif
  71 xsum3 (size_t size1, size_t size2, size_t size3)
  72 {
  73   return xsum (xsum (size1, size2), size3);
  74 }
  75
  76 /* Sum of four sizes, with overflow check.  */
  77 XSIZE_INLINE size_t
  78 #if __GNUC__ >= 3
  79 __attribute__ ((__pure__))
  80 #endif
  81 xsum4 (size_t size1, size_t size2, size_t size3, size_t size4)
  82 {
  83   return xsum (xsum (xsum (size1, size2), size3), size4);
  84 }
  85
  86 /* Maximum of two sizes, with overflow check.  */
  87 XSIZE_INLINE size_t
  88 #if __GNUC__ >= 3
  89 __attribute__ ((__pure__))
  90 #endif
  91 xmax (size_t size1, size_t size2)
  92 {
  93   /* No explicit check is needed here, because for any n:
  94      max (SIZE_MAX, n) == SIZE_MAX and max (n, SIZE_MAX) == SIZE_MAX.  */
  95   return (size1 >= size2 ? size1 : size2);
  96 }
  97
  98 /* Multiplication of a count with an element size, with overflow check.
  99    The count must be >= 0 and the element size must be > 0.
 100    This is a macro, not a function, so that it works correctly even
 101    when N is of a wider type and N > SIZE_MAX.  */
 102 #define xtimes(N, ELSIZE) \
 103   ((N) <= SIZE_MAX / (ELSIZE) ? (size_t) (N) * (ELSIZE) : SIZE_MAX)
 104
 105 /* Check for overflow.  */
 106 #define size_overflow_p(SIZE) \
 107   ((SIZE) == SIZE_MAX)
 108 /* Check against overflow.  */
 109 #define size_in_bounds_p(SIZE) \
 110   ((SIZE) != SIZE_MAX)
 111
 112 _GL_INLINE_HEADER_END
 113
 114 #endif /* _XSIZE_H */