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135 lines
5.0 KiB
C
135 lines
5.0 KiB
C
/* A type for indices and sizes.
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Copyright (C) 2020-2023 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library 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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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#ifndef _IDX_H
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#define _IDX_H
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/* Get ptrdiff_t. */
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#include <stddef.h>
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/* Get PTRDIFF_MAX. */
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#include <stdint.h>
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/* The type 'idx_t' holds an (array) index or an (object) size.
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Its implementation promotes to a signed integer type,
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which can hold the values
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0..2^63-1 (on 64-bit platforms) or
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0..2^31-1 (on 32-bit platforms).
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Why a signed integer type?
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* Security: Signed types can be checked for overflow via
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'-fsanitize=undefined', but unsigned types cannot.
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* Comparisons without surprises: ISO C99 § 6.3.1.8 specifies a few
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surprising results for comparisons, such as
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(int) -3 < (unsigned long) 7 => false
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(int) -3 < (unsigned int) 7 => false
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and on 32-bit machines:
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(long) -3 < (unsigned int) 7 => false
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This is surprising because the natural comparison order is by
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value in the realm of infinite-precision signed integers (ℤ).
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The best way to get rid of such surprises is to use signed types
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for numerical integer values, and use unsigned types only for
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bit masks and enums.
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Why not use 'size_t' directly?
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* Because 'size_t' is an unsigned type, and a signed type is better.
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See above.
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Why not use 'ssize_t'?
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* 'ptrdiff_t' is more portable; it is standardized by ISO C
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whereas 'ssize_t' is standardized only by POSIX.
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* 'ssize_t' is not required to be as wide as 'size_t', and some
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now-obsolete POSIX platforms had 'size_t' wider than 'ssize_t'.
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* Conversely, some now-obsolete platforms had 'ptrdiff_t' wider
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than 'size_t', which can be a win and conforms to POSIX.
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Won't this cause a problem with objects larger than PTRDIFF_MAX?
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* Typical modern or large platforms do not allocate such objects,
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so this is not much of a problem in practice; for example, you
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can safely write 'idx_t len = strlen (s);'. To port to older
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small platforms where allocations larger than PTRDIFF_MAX could
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in theory be a problem, you can use Gnulib's ialloc module, or
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functions like ximalloc in Gnulib's xalloc module.
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Why not use 'ptrdiff_t' directly?
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* Maintainability: When reading and modifying code, it helps to know that
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a certain variable cannot have negative values. For example, when you
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have a loop
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int n = ...;
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for (int i = 0; i < n; i++) ...
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or
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ptrdiff_t n = ...;
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for (ptrdiff_t i = 0; i < n; i++) ...
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you have to ask yourself "what if n < 0?". Whereas in
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idx_t n = ...;
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for (idx_t i = 0; i < n; i++) ...
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you know that this case cannot happen.
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Similarly, when a programmer writes
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idx_t = ptr2 - ptr1;
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there is an implied assertion that ptr1 and ptr2 point into the same
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object and that ptr1 <= ptr2.
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* Being future-proof: In the future, range types (integers which are
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constrained to a certain range of values) may be added to C compilers
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or to the C standard. Several programming languages (Ada, Haskell,
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Common Lisp, Pascal) already have range types. Such range types may
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help producing good code and good warnings. The type 'idx_t' could
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then be typedef'ed to a range type that is signed after promotion. */
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/* In the future, idx_t could be typedef'ed to a signed range type.
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The clang "extended integer types", supported in Clang 11 or newer
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<https://clang.llvm.org/docs/LanguageExtensions.html#extended-integer-types>,
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are a special case of range types. However, these types don't support binary
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operators with plain integer types (e.g. expressions such as x > 1).
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Therefore, they don't behave like signed types (and not like unsigned types
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either). So, we cannot use them here. */
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/* Use the signed type 'ptrdiff_t'. */
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/* Note: ISO C does not mandate that 'size_t' and 'ptrdiff_t' have the same
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size, but it is so on all platforms we have seen since 1990. */
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typedef ptrdiff_t idx_t;
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/* IDX_MAX is the maximum value of an idx_t. */
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#define IDX_MAX PTRDIFF_MAX
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/* So far no need has been found for an IDX_WIDTH macro.
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Perhaps there should be another macro IDX_VALUE_BITS that does not
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count the sign bit and is therefore one less than PTRDIFF_WIDTH. */
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#endif /* _IDX_H */
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