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/* xsize.h -- Checked size_t computations.
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Copyright (C) 2003 Free Software Foundation, Inc.
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This program is free software; you can redistribute it and/or modify it
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under the terms of the GNU Library General Public License as published
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by the Free Software Foundation; either version 2, or (at your option)
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Library General Public License for more details.
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You should have received a copy of the GNU Library General Public
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License along with this program; if not, write to the Free Software
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Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
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/* The size of memory objects is often computed through expressions of
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void* p = malloc (header_size + n * element_size).
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These computations can lead to overflow. When this happens, malloc()
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returns a piece of memory that is way too small, and the program then
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crashes while attempting to fill the memory.
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To avoid this, the functions and macros in this file check for overflow.
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The convention is that SIZE_MAX represents overflow.
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malloc (SIZE_MAX) is not guaranteed to fail -- think of a malloc
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implementation that uses mmap --, it's recommended to use size_overflow_p()
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or size_in_bounds_p() before invoking malloc().
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The example thus becomes:
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size_t size = xsum (header_size, xtimes (n, element_size));
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void *p = (size_in_bounds_p (size) ? malloc (size) : NULL);
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/* Convert an arbitrary value >= 0 to type size_t. */
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#define xcast_size_t(N) \
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((N) <= SIZE_MAX ? (size_t) (N) : SIZE_MAX)
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/* Sum of two sizes, with overflow check. */
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__attribute__ ((__pure__))
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xsum (size_t size1, size_t size2)
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size_t sum = size1 + size2;
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return (sum >= size1 ? sum : SIZE_MAX);
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/* Sum of three sizes, with overflow check. */
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__attribute__ ((__pure__))
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xsum3 (size_t size1, size_t size2, size_t size3)
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return xsum (xsum (size1, size2), size3);
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/* Sum of four sizes, with overflow check. */
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__attribute__ ((__pure__))
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xsum4 (size_t size1, size_t size2, size_t size3, size_t size4)
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return xsum (xsum (xsum (size1, size2), size3), size4);
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/* Maximum of two sizes, with overflow check. */
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__attribute__ ((__pure__))
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xmax (size_t size1, size_t size2)
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/* No explicit check is needed here, because for any n:
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max (SIZE_MAX, n) == SIZE_MAX and max (n, SIZE_MAX) == SIZE_MAX. */
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return (size1 >= size2 ? size1 : size2);
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/* Multiplication of a count with an element size, with overflow check.
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The count must be >= 0 and the element size must be > 0.
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This is a macro, not an inline function, so that it works correctly even
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when N is of a wider tupe and N > SIZE_MAX. */
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#define xtimes(N, ELSIZE) \
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((N) <= SIZE_MAX / (ELSIZE) ? (size_t) (N) * (ELSIZE) : SIZE_MAX)
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/* Check for overflow. */
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#define size_overflow_p(SIZE) \
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/* Check against overflow. */
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#define size_in_bounds_p(SIZE) \
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#endif /* _XSIZE_H */
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intl/xsize.h
