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 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 /* The size of memory objects is often computed through expressions of
  44    type size_t. Example:
  45       void* p = malloc (header_size + n * element_size).
  46    These computations can lead to overflow.  When this happens, malloc()
  47    returns a piece of memory that is way too small, and the program then
  48    crashes while attempting to fill the memory.
  49    To avoid this, the functions and macros in this file check for overflow.
  50    The convention is that SIZE_MAX represents overflow.
  51    malloc (SIZE_MAX) is not guaranteed to fail -- think of a malloc
  52    implementation that uses mmap --, it's recommended to use size_overflow_p()
  53    or size_in_bounds_p() before invoking malloc().
  54    The example thus becomes:
  55       size_t size = xsum (header_size, xtimes (n, element_size));
  56       void *p = (size_in_bounds_p (size) ? malloc (size) : NULL);
  57 */
  58
  59 /* Convert an arbitrary value >= 0 to type size_t.  */
  60 #define xcast_size_t(N) \
  61   ((N) <= SIZE_MAX ? (size_t) (N) : SIZE_MAX)
  62
  63 /* Sum of two sizes, with overflow check.  */
  64 XSIZE_INLINE size_t ATTRIBUTE_PURE
  65 xsum (size_t size1, size_t size2)
  66 {
  67   size_t sum = size1 + size2;
  68   return (sum >= size1 ? sum : SIZE_MAX);
  69 }
  70
  71 /* Sum of three sizes, with overflow check.  */
  72 XSIZE_INLINE size_t ATTRIBUTE_PURE
  73 xsum3 (size_t size1, size_t size2, size_t size3)
  74 {
  75   return xsum (xsum (size1, size2), size3);
  76 }
  77
  78 /* Sum of four sizes, with overflow check.  */
  79 XSIZE_INLINE size_t ATTRIBUTE_PURE
  80 xsum4 (size_t size1, size_t size2, size_t size3, size_t size4)
  81 {
  82   return xsum (xsum (xsum (size1, size2), size3), size4);
  83 }
  84
  85 /* Maximum of two sizes, with overflow check.  */
  86 XSIZE_INLINE size_t ATTRIBUTE_PURE
  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 a 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 _GL_INLINE_HEADER_END
 109
 110 #endif /* _XSIZE_H */