Theodore Ts'o | b0cacab | 2004-11-30 19:00:19 -0500 | [diff] [blame] | 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 it |
| 6 | under the terms of the GNU Library General Public License as published |
| 7 | by 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 GNU |
| 13 | Library General Public License for more details. |
| 14 | |
| 15 | You should have received a copy of the GNU Library General Public |
| 16 | License along with this program; if not, write to the Free Software |
| 17 | Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, |
| 18 | USA. */ |
| 19 | |
| 20 | #ifndef _XSIZE_H |
| 21 | #define _XSIZE_H |
| 22 | |
| 23 | /* Get size_t. */ |
| 24 | #include <stddef.h> |
| 25 | |
| 26 | /* Get SIZE_MAX. */ |
| 27 | #include <limits.h> |
| 28 | #if HAVE_STDINT_H |
| 29 | # include <stdint.h> |
| 30 | #endif |
| 31 | |
| 32 | /* The size of memory objects is often computed through expressions of |
| 33 | type size_t. Example: |
| 34 | void* p = malloc (header_size + n * element_size). |
| 35 | These computations can lead to overflow. When this happens, malloc() |
| 36 | returns a piece of memory that is way too small, and the program then |
| 37 | crashes while attempting to fill the memory. |
| 38 | To avoid this, the functions and macros in this file check for overflow. |
| 39 | The convention is that SIZE_MAX represents overflow. |
| 40 | malloc (SIZE_MAX) is not guaranteed to fail -- think of a malloc |
| 41 | implementation that uses mmap --, it's recommended to use size_overflow_p() |
| 42 | or size_in_bounds_p() before invoking malloc(). |
| 43 | The example thus becomes: |
| 44 | size_t size = xsum (header_size, xtimes (n, element_size)); |
| 45 | void *p = (size_in_bounds_p (size) ? malloc (size) : NULL); |
| 46 | */ |
| 47 | |
| 48 | /* Convert an arbitrary value >= 0 to type size_t. */ |
| 49 | #define xcast_size_t(N) \ |
| 50 | ((N) <= SIZE_MAX ? (size_t) (N) : SIZE_MAX) |
| 51 | |
| 52 | /* Sum of two sizes, with overflow check. */ |
| 53 | static inline size_t |
| 54 | #if __GNUC__ >= 3 |
| 55 | __attribute__ ((__pure__)) |
| 56 | #endif |
| 57 | xsum (size_t size1, size_t size2) |
| 58 | { |
| 59 | size_t sum = size1 + size2; |
| 60 | return (sum >= size1 ? sum : SIZE_MAX); |
| 61 | } |
| 62 | |
| 63 | /* Sum of three sizes, with overflow check. */ |
| 64 | static inline size_t |
| 65 | #if __GNUC__ >= 3 |
| 66 | __attribute__ ((__pure__)) |
| 67 | #endif |
| 68 | xsum3 (size_t size1, size_t size2, size_t size3) |
| 69 | { |
| 70 | return xsum (xsum (size1, size2), size3); |
| 71 | } |
| 72 | |
| 73 | /* Sum of four sizes, with overflow check. */ |
| 74 | static inline size_t |
| 75 | #if __GNUC__ >= 3 |
| 76 | __attribute__ ((__pure__)) |
| 77 | #endif |
| 78 | xsum4 (size_t size1, size_t size2, size_t size3, size_t size4) |
| 79 | { |
| 80 | return xsum (xsum (xsum (size1, size2), size3), size4); |
| 81 | } |
| 82 | |
| 83 | /* Maximum of two sizes, with overflow check. */ |
| 84 | static inline size_t |
| 85 | #if __GNUC__ >= 3 |
| 86 | __attribute__ ((__pure__)) |
| 87 | #endif |
| 88 | xmax (size_t size1, size_t size2) |
| 89 | { |
| 90 | /* No explicit check is needed here, because for any n: |
| 91 | max (SIZE_MAX, n) == SIZE_MAX and max (n, SIZE_MAX) == SIZE_MAX. */ |
| 92 | return (size1 >= size2 ? size1 : size2); |
| 93 | } |
| 94 | |
| 95 | /* Multiplication of a count with an element size, with overflow check. |
| 96 | The count must be >= 0 and the element size must be > 0. |
| 97 | This is a macro, not an inline function, so that it works correctly even |
| 98 | when N is of a wider tupe and N > SIZE_MAX. */ |
| 99 | #define xtimes(N, ELSIZE) \ |
| 100 | ((N) <= SIZE_MAX / (ELSIZE) ? (size_t) (N) * (ELSIZE) : SIZE_MAX) |
| 101 | |
| 102 | /* Check for overflow. */ |
| 103 | #define size_overflow_p(SIZE) \ |
| 104 | ((SIZE) == SIZE_MAX) |
| 105 | /* Check against overflow. */ |
| 106 | #define size_in_bounds_p(SIZE) \ |
| 107 | ((SIZE) != SIZE_MAX) |
| 108 | |
| 109 | #endif /* _XSIZE_H */ |