lib/xsize.h

   1 /* xsize.h -- Checked size_t computations.
   2
   3    Copyright (C) 2003, 2008-2024 Free Software Foundation, Inc.
   4
   5    This file is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU Lesser General Public License as
   7    published by the Free Software Foundation; either version 2.1 of the
   8    License, or (at your option) any later version.
   9
  10    This file 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, see <https://www.gnu.org/licenses/>.  */
  17
  18 #ifndef _XSIZE_H
  19 #define _XSIZE_H
  20
  21 /* This file uses _GL_INLINE_HEADER_BEGIN, _GL_INLINE, HAVE_STDINT_H.  */
  22 #if !_GL_CONFIG_H_INCLUDED
  23  #error "Please include config.h first."
  24 #endif
  25
  26 /* Get size_t.  */
  27 #include <stddef.h>
  28
  29 /* Get INT_MAX, SIZE_MAX.  */
  30 #include <limits.h>
  31 #if HAVE_STDINT_H
  32 # include <stdint.h>
  33 #endif
  34
  35 /* Get ATTRIBUTE_PURE.  */
  36 #include "attribute.h"
  37
  38 _GL_INLINE_HEADER_BEGIN
  39 #ifndef XSIZE_INLINE
  40 # define XSIZE_INLINE _GL_INLINE
  41 #endif
  42
  43 #ifdef __cplusplus
  44 extern "C" {
  45 #endif
  46
  47
  48 /* The size of memory objects is often computed through expressions of
  49    type size_t. Example:
  50       void* p = malloc (header_size + n * element_size).
  51    These computations can lead to overflow.  When this happens, malloc()
  52    returns a piece of memory that is way too small, and the program then
  53    crashes while attempting to fill the memory.
  54    To avoid this, the functions and macros in this file check for overflow.
  55    The convention is that SIZE_MAX represents overflow.
  56    malloc (SIZE_MAX) is not guaranteed to fail -- think of a malloc
  57    implementation that uses mmap --, it's recommended to use size_overflow_p()
  58    or size_in_bounds_p() before invoking malloc().
  59    The example thus becomes:
  60       size_t size = xsum (header_size, xtimes (n, element_size));
  61       void *p = (size_in_bounds_p (size) ? malloc (size) : NULL);
  62 */
  63
  64 /* Convert an arbitrary N >= 0 to type size_t.
  65    N should not have side effects.  */
  66 #define xcast_size_t(N) \
  67   ((N) <= SIZE_MAX ? (size_t) (N) : SIZE_MAX)
  68
  69 /* Sum of two sizes, with overflow check.  */
  70 XSIZE_INLINE size_t ATTRIBUTE_PURE
  71 xsum (size_t size1, size_t size2)
  72 {
  73   if (INT_MAX < SIZE_MAX)
  74     {
  75       /* Optimize for the common case where size_t arithmetic wraps
  76          around without undefined behavior.  */
  77       size_t sum = size1 + size2;
  78       return size1 <= sum ? sum : SIZE_MAX;
  79     }
  80
  81   return size1 <= SIZE_MAX - size2 ? size1 + size2 : SIZE_MAX;
  82 }
  83
  84 /* Sum of three sizes, with overflow check.  */
  85 XSIZE_INLINE size_t ATTRIBUTE_PURE
  86 xsum3 (size_t size1, size_t size2, size_t size3)
  87 {
  88   return xsum (xsum (size1, size2), size3);
  89 }
  90
  91 /* Sum of four sizes, with overflow check.  */
  92 XSIZE_INLINE size_t ATTRIBUTE_PURE
  93 xsum4 (size_t size1, size_t size2, size_t size3, size_t size4)
  94 {
  95   return xsum (xsum (xsum (size1, size2), size3), size4);
  96 }
  97
  98 /* Maximum of two sizes, with overflow check.  */
  99 XSIZE_INLINE size_t ATTRIBUTE_PURE
 100 xmax (size_t size1, size_t size2)
 101 {
 102   /* No explicit check is needed here, because for any n:
 103      max (SIZE_MAX, n) == SIZE_MAX and max (n, SIZE_MAX) == SIZE_MAX.  */
 104   return (size1 >= size2 ? size1 : size2);
 105 }
 106
 107 /* Multiplication of a count with an element size, with overflow check.
 108    The count must be >= 0 and the element size must be > 0.
 109    Arguments should not have side effects.
 110    The element size's type should be no wider than size_t.
 111    This is a macro, not a function, so that it works correctly even
 112    when N is of a wider type and N > SIZE_MAX.  */
 113 #define xtimes(N, ELSIZE) \
 114   ((N) <= SIZE_MAX / (ELSIZE) ? (size_t) (N) * (ELSIZE) : SIZE_MAX)
 115
 116 /* Check for overflow.  */
 117 #define size_overflow_p(SIZE) \
 118   ((SIZE) == SIZE_MAX)
 119 /* Check against overflow.  */
 120 #define size_in_bounds_p(SIZE) \
 121   ((SIZE) != SIZE_MAX)
 122
 123
 124 #ifdef __cplusplus
 125 }
 126 #endif
 127
 128 _GL_INLINE_HEADER_END
 129
 130 #endif /* _XSIZE_H */