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434
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1 /* xsize.h -- Checked size_t computations.
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2
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3 Copyright (C) 2003, 2008, 2009, 2010 Free Software Foundation, Inc.
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4
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5 This program is free software; you can redistribute it and/or modify
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6 it under the terms of the GNU General Public License as published by
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7 the Free Software Foundation; either version 3, or (at your option)
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8 any later version.
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9
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10 This program is distributed in the hope that it will be useful,
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11 but WITHOUT ANY WARRANTY; without even the implied warranty of
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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13 GNU General Public License for more details.
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14
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15 You should have received a copy of the GNU General Public License
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16 along with this program; if not, write to the Free Software Foundation,
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17 Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
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18
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19 #ifndef _XSIZE_H
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20 #define _XSIZE_H
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21
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22 /* Get size_t. */
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23 #include <stddef.h>
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24
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25 /* Get SIZE_MAX. */
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26 #include <limits.h>
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27 #if HAVE_STDINT_H
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28 # include <stdint.h>
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29 #endif
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30
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31 /* The size of memory objects is often computed through expressions of
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32 type size_t. Example:
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33 void* p = malloc (header_size + n * element_size).
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34 These computations can lead to overflow. When this happens, malloc()
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35 returns a piece of memory that is way too small, and the program then
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36 crashes while attempting to fill the memory.
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37 To avoid this, the functions and macros in this file check for overflow.
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38 The convention is that SIZE_MAX represents overflow.
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39 malloc (SIZE_MAX) is not guaranteed to fail -- think of a malloc
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40 implementation that uses mmap --, it's recommended to use size_overflow_p()
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41 or size_in_bounds_p() before invoking malloc().
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42 The example thus becomes:
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43 size_t size = xsum (header_size, xtimes (n, element_size));
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44 void *p = (size_in_bounds_p (size) ? malloc (size) : NULL);
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45 */
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46
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47 /* Convert an arbitrary value >= 0 to type size_t. */
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48 #define xcast_size_t(N) \
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49 ((N) <= SIZE_MAX ? (size_t) (N) : SIZE_MAX)
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50
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51 /* Sum of two sizes, with overflow check. */
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52 static inline size_t
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53 #if __GNUC__ >= 3
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54 __attribute__ ((__pure__))
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55 #endif
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56 xsum (size_t size1, size_t size2)
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57 {
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58 size_t sum = size1 + size2;
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59 return (sum >= size1 ? sum : SIZE_MAX);
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60 }
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61
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62 /* Sum of three sizes, with overflow check. */
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63 static inline size_t
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64 #if __GNUC__ >= 3
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65 __attribute__ ((__pure__))
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66 #endif
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67 xsum3 (size_t size1, size_t size2, size_t size3)
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68 {
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69 return xsum (xsum (size1, size2), size3);
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70 }
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71
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72 /* Sum of four sizes, with overflow check. */
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73 static inline size_t
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74 #if __GNUC__ >= 3
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75 __attribute__ ((__pure__))
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76 #endif
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77 xsum4 (size_t size1, size_t size2, size_t size3, size_t size4)
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78 {
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79 return xsum (xsum (xsum (size1, size2), size3), size4);
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80 }
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81
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82 /* Maximum of two sizes, with overflow check. */
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83 static inline size_t
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84 #if __GNUC__ >= 3
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85 __attribute__ ((__pure__))
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86 #endif
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87 xmax (size_t size1, size_t size2)
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88 {
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89 /* No explicit check is needed here, because for any n:
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90 max (SIZE_MAX, n) == SIZE_MAX and max (n, SIZE_MAX) == SIZE_MAX. */
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91 return (size1 >= size2 ? size1 : size2);
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92 }
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93
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94 /* Multiplication of a count with an element size, with overflow check.
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95 The count must be >= 0 and the element size must be > 0.
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96 This is a macro, not an inline function, so that it works correctly even
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97 when N is of a wider type and N > SIZE_MAX. */
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98 #define xtimes(N, ELSIZE) \
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99 ((N) <= SIZE_MAX / (ELSIZE) ? (size_t) (N) * (ELSIZE) : SIZE_MAX)
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100
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101 /* Check for overflow. */
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102 #define size_overflow_p(SIZE) \
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103 ((SIZE) == SIZE_MAX)
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104 /* Check against overflow. */
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105 #define size_in_bounds_p(SIZE) \
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106 ((SIZE) != SIZE_MAX)
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107
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108 #endif /* _XSIZE_H */
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