1 /* xsize.h -- Checked size_t computations. 2 3 Copyright (C) 2003 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 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 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, 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 tupe 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 */ 109