mirror of
https://git.savannah.gnu.org/git/emacs.git
synced 2024-11-25 07:28:20 +00:00
135 lines
5.0 KiB
C
135 lines
5.0 KiB
C
/* A type for indices and sizes.
|
||
Copyright (C) 2020-2023 Free Software Foundation, Inc.
|
||
This file is part of the GNU C Library.
|
||
|
||
The GNU C Library is free software; you can redistribute it and/or
|
||
modify it under the terms of the GNU Lesser General Public
|
||
License as published by the Free Software Foundation; either
|
||
version 2.1 of the License, or (at your option) any later version.
|
||
|
||
The GNU C Library is distributed in the hope that it will be useful,
|
||
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
||
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
||
Lesser General Public License for more details.
|
||
|
||
You should have received a copy of the GNU Lesser General Public
|
||
License along with the GNU C Library; if not, see
|
||
<https://www.gnu.org/licenses/>. */
|
||
|
||
#ifndef _IDX_H
|
||
#define _IDX_H
|
||
|
||
/* Get ptrdiff_t. */
|
||
#include <stddef.h>
|
||
|
||
/* Get PTRDIFF_MAX. */
|
||
#include <stdint.h>
|
||
|
||
/* The type 'idx_t' holds an (array) index or an (object) size.
|
||
Its implementation promotes to a signed integer type,
|
||
which can hold the values
|
||
0..2^63-1 (on 64-bit platforms) or
|
||
0..2^31-1 (on 32-bit platforms).
|
||
|
||
Why a signed integer type?
|
||
|
||
* Security: Signed types can be checked for overflow via
|
||
'-fsanitize=undefined', but unsigned types cannot.
|
||
|
||
* Comparisons without surprises: ISO C99 § 6.3.1.8 specifies a few
|
||
surprising results for comparisons, such as
|
||
|
||
(int) -3 < (unsigned long) 7 => false
|
||
(int) -3 < (unsigned int) 7 => false
|
||
and on 32-bit machines:
|
||
(long) -3 < (unsigned int) 7 => false
|
||
|
||
This is surprising because the natural comparison order is by
|
||
value in the realm of infinite-precision signed integers (ℤ).
|
||
|
||
The best way to get rid of such surprises is to use signed types
|
||
for numerical integer values, and use unsigned types only for
|
||
bit masks and enums.
|
||
|
||
Why not use 'size_t' directly?
|
||
|
||
* Because 'size_t' is an unsigned type, and a signed type is better.
|
||
See above.
|
||
|
||
Why not use 'ssize_t'?
|
||
|
||
* 'ptrdiff_t' is more portable; it is standardized by ISO C
|
||
whereas 'ssize_t' is standardized only by POSIX.
|
||
|
||
* 'ssize_t' is not required to be as wide as 'size_t', and some
|
||
now-obsolete POSIX platforms had 'size_t' wider than 'ssize_t'.
|
||
|
||
* Conversely, some now-obsolete platforms had 'ptrdiff_t' wider
|
||
than 'size_t', which can be a win and conforms to POSIX.
|
||
|
||
Won't this cause a problem with objects larger than PTRDIFF_MAX?
|
||
|
||
* Typical modern or large platforms do not allocate such objects,
|
||
so this is not much of a problem in practice; for example, you
|
||
can safely write 'idx_t len = strlen (s);'. To port to older
|
||
small platforms where allocations larger than PTRDIFF_MAX could
|
||
in theory be a problem, you can use Gnulib's ialloc module, or
|
||
functions like ximalloc in Gnulib's xalloc module.
|
||
|
||
Why not use 'ptrdiff_t' directly?
|
||
|
||
* Maintainability: When reading and modifying code, it helps to know that
|
||
a certain variable cannot have negative values. For example, when you
|
||
have a loop
|
||
|
||
int n = ...;
|
||
for (int i = 0; i < n; i++) ...
|
||
|
||
or
|
||
|
||
ptrdiff_t n = ...;
|
||
for (ptrdiff_t i = 0; i < n; i++) ...
|
||
|
||
you have to ask yourself "what if n < 0?". Whereas in
|
||
|
||
idx_t n = ...;
|
||
for (idx_t i = 0; i < n; i++) ...
|
||
|
||
you know that this case cannot happen.
|
||
|
||
Similarly, when a programmer writes
|
||
|
||
idx_t = ptr2 - ptr1;
|
||
|
||
there is an implied assertion that ptr1 and ptr2 point into the same
|
||
object and that ptr1 <= ptr2.
|
||
|
||
* Being future-proof: In the future, range types (integers which are
|
||
constrained to a certain range of values) may be added to C compilers
|
||
or to the C standard. Several programming languages (Ada, Haskell,
|
||
Common Lisp, Pascal) already have range types. Such range types may
|
||
help producing good code and good warnings. The type 'idx_t' could
|
||
then be typedef'ed to a range type that is signed after promotion. */
|
||
|
||
/* In the future, idx_t could be typedef'ed to a signed range type.
|
||
The clang "extended integer types", supported in Clang 11 or newer
|
||
<https://clang.llvm.org/docs/LanguageExtensions.html#extended-integer-types>,
|
||
are a special case of range types. However, these types don't support binary
|
||
operators with plain integer types (e.g. expressions such as x > 1).
|
||
Therefore, they don't behave like signed types (and not like unsigned types
|
||
either). So, we cannot use them here. */
|
||
|
||
/* Use the signed type 'ptrdiff_t'. */
|
||
/* Note: ISO C does not mandate that 'size_t' and 'ptrdiff_t' have the same
|
||
size, but it is so on all platforms we have seen since 1990. */
|
||
typedef ptrdiff_t idx_t;
|
||
|
||
/* IDX_MAX is the maximum value of an idx_t. */
|
||
#define IDX_MAX PTRDIFF_MAX
|
||
|
||
/* So far no need has been found for an IDX_WIDTH macro.
|
||
Perhaps there should be another macro IDX_VALUE_BITS that does not
|
||
count the sign bit and is therefore one less than PTRDIFF_WIDTH. */
|
||
|
||
#endif /* _IDX_H */
|