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       1                 :            : /* xsize.h -- Checked size_t computations.
       2                 :            : 
       3                 :            :    Copyright (C) 2003, 2008, 2009, 2010 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 3, 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                 :          0 : xsum (size_t size1, size_t size2)
      57                 :            : {
      58                 :          0 :   size_t sum = size1 + size2;
      59         [ #  # ]:          0 :   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                 :          0 : xsum4 (size_t size1, size_t size2, size_t size3, size_t size4)
      78                 :            : {
      79                 :          0 :   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                 :          0 : 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                 :          0 :   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 */