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1 | /* xsize.h -- Checked size_t computations. |
2 | ||
f0007cad | 3 | Copyright (C) 2003, 2008-2012 Free Software Foundation, Inc. |
c4b681fd LC |
4 | |
5 | This program is free software; you can redistribute it and/or modify | |
6 | it under the terms of the GNU Lesser General Public License as published by | |
7 | the Free Software Foundation; either version 2, 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 Lesser General Public License for more details. | |
14 | ||
15 | You should have received a copy of the GNU Lesser General Public License | |
16 | along with this program; if not, write to the Free Software Foundation, | |
17 | Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ | |
18 | ||
19 | #ifndef _XSIZE_H | |
20 | #define _XSIZE_H | |
21 | ||
22 | /* Get size_t. */ | |
23 | #include <stddef.h> | |
24 | ||
25 | /* Get SIZE_MAX. */ | |
26 | #include <limits.h> | |
27 | #if HAVE_STDINT_H | |
28 | # include <stdint.h> | |
29 | #endif | |
30 | ||
31 | /* The size of memory objects is often computed through expressions of | |
32 | type size_t. Example: | |
33 | void* p = malloc (header_size + n * element_size). | |
34 | These computations can lead to overflow. When this happens, malloc() | |
35 | returns a piece of memory that is way too small, and the program then | |
36 | crashes while attempting to fill the memory. | |
37 | To avoid this, the functions and macros in this file check for overflow. | |
38 | The convention is that SIZE_MAX represents overflow. | |
39 | malloc (SIZE_MAX) is not guaranteed to fail -- think of a malloc | |
40 | implementation that uses mmap --, it's recommended to use size_overflow_p() | |
41 | or size_in_bounds_p() before invoking malloc(). | |
42 | The example thus becomes: | |
43 | size_t size = xsum (header_size, xtimes (n, element_size)); | |
44 | void *p = (size_in_bounds_p (size) ? malloc (size) : NULL); | |
45 | */ | |
46 | ||
47 | /* Convert an arbitrary value >= 0 to type size_t. */ | |
48 | #define xcast_size_t(N) \ | |
49 | ((N) <= SIZE_MAX ? (size_t) (N) : SIZE_MAX) | |
50 | ||
51 | /* Sum of two sizes, with overflow check. */ | |
52 | static inline size_t | |
53 | #if __GNUC__ >= 3 | |
54 | __attribute__ ((__pure__)) | |
55 | #endif | |
56 | xsum (size_t size1, size_t size2) | |
57 | { | |
58 | size_t sum = size1 + size2; | |
59 | return (sum >= size1 ? sum : SIZE_MAX); | |
60 | } | |
61 | ||
62 | /* Sum of three sizes, with overflow check. */ | |
63 | static inline size_t | |
64 | #if __GNUC__ >= 3 | |
65 | __attribute__ ((__pure__)) | |
66 | #endif | |
67 | xsum3 (size_t size1, size_t size2, size_t size3) | |
68 | { | |
69 | return xsum (xsum (size1, size2), size3); | |
70 | } | |
71 | ||
72 | /* Sum of four sizes, with overflow check. */ | |
73 | static inline size_t | |
74 | #if __GNUC__ >= 3 | |
75 | __attribute__ ((__pure__)) | |
76 | #endif | |
77 | xsum4 (size_t size1, size_t size2, size_t size3, size_t size4) | |
78 | { | |
79 | return xsum (xsum (xsum (size1, size2), size3), size4); | |
80 | } | |
81 | ||
82 | /* Maximum of two sizes, with overflow check. */ | |
83 | static inline size_t | |
84 | #if __GNUC__ >= 3 | |
85 | __attribute__ ((__pure__)) | |
86 | #endif | |
87 | xmax (size_t size1, size_t size2) | |
88 | { | |
89 | /* No explicit check is needed here, because for any n: | |
90 | max (SIZE_MAX, n) == SIZE_MAX and max (n, SIZE_MAX) == SIZE_MAX. */ | |
91 | return (size1 >= size2 ? size1 : size2); | |
92 | } | |
93 | ||
94 | /* Multiplication of a count with an element size, with overflow check. | |
95 | The count must be >= 0 and the element size must be > 0. | |
96 | This is a macro, not an inline function, so that it works correctly even | |
97 | when N is of a wider type and N > SIZE_MAX. */ | |
98 | #define xtimes(N, ELSIZE) \ | |
99 | ((N) <= SIZE_MAX / (ELSIZE) ? (size_t) (N) * (ELSIZE) : SIZE_MAX) | |
100 | ||
101 | /* Check for overflow. */ | |
102 | #define size_overflow_p(SIZE) \ | |
103 | ((SIZE) == SIZE_MAX) | |
104 | /* Check against overflow. */ | |
105 | #define size_in_bounds_p(SIZE) \ | |
106 | ((SIZE) != SIZE_MAX) | |
107 | ||
108 | #endif /* _XSIZE_H */ |