Appendix F — What C23 added, and where this book covers it

Every edition of the standard records “what changed” about itself (C23′s annex M.2). Taking that list as the frame, this appendix gathers where in this book each item is covered. It is a place for someone who already knows the new standard to ask “which chapter tells that story”, and equally for someone who has finished the book to check “did I meet all of C23”.

★ This book does not teach C23 separately. Rather than teaching the old way first and appending the new, it explains in C23 from the start and adds “it used to be like this” only where that helps. So the items below do not sit together; they are scattered through their own subjects. The three tables below stitch those places back together.

The language#

What	In brief	Where
bool, true, false and others promoted to keywords	what were macros became keywords	chapter 97
nullptr and nullptr_t	a null constant with a type of its own	chapters 37 and 97
constexpr	an object whose value is fixed at translation time	chapters 21 and 97
typeof, typeof_unqual	write down an expression’s type verbatim	chapter 97
auto type inference	the type is drawn from the initialiser	chapter 97
attributes [[…]]	write intent no type can hold (seven of them)	chapter 97
_BitInt(N)	an integer whose width you choose	chapters 21 and 28
two’s complement settled, old sign forms removed	integer representation pinned to one	chapters 6 and 28
limits redefined in terms of width	the negative end of the guaranteed range moved	chapter 28
binary literals 0b	write bits as bits	chapters 21 and 53
digit separator '	1'000'000 reads in groups	chapter 21
empty initialiser {}	initialisation that zeroes the padding too	chapter 47
storage class on compound literals	their lifetime can be changed	chapter 48
fixed underlying type for enums	enum E : unsigned { … }	chapter 27
relaxed tag compatibility	same-tag, same-content structs are compatible within one translation unit too	chapter 47
labels in more places	before declarations and at a block’s end	chapter 32
K&R identifier-list definitions removed	the old definition syntax is gone	chapter 66
f() = f(void)	empty parentheses now mean no parameters	chapter 66
parameter names may be omitted	even in a definition	chapter 66
va_start needs no second argument	... may stand with no parameter before it	chapter 67
variably modified types mandated	VLAs themselves stay optional	chapter 40
Unicode identifiers	which characters may appear in a name is settled	chapter 8
@, $ and the grave accent (U+0060) added to the character set	more characters may appear in source	chapter 8
u8 character constants	the string prefix now works on characters	chapter 21
u8, u, U encodings pinned	settled as UTF-8, UTF-16, UTF-32	chapters 8 and 85
literal encodings separated	split apart from the execution character set	chapter 8
wide/narrow literal concatenation removed	mixing them is gone	chapter 43
potentially reserved identifiers	a middle level between reserved and not	chapter 63
ptrdiff_t minimum width 16	the requirement fell from 17	chapter 27
width exception for extended integer types	allows things like int128_t	chapter 28

Table 115.1 — What C23 changed in the language

The preprocessor#

What	In brief	Where
#embed	bring binary data into the source as it is	chapter 65
__VA_OPT__	handles the empty variadic case	chapter 65
__has_include	asks whether a header is there	chapters 53 and 96
#elifdef, #elifndef	the short form of #elif defined	chapter 65
#warning	says something without failing	chapter 65
one-argument static_assert	the message may be left out	chapter 90

Table 115.2 — What C23 added to the preprocessor

The library#

What	In brief	Where
<stdbit.h>	counting and rotating bits became standard functions	chapters 53 and 93
<stdckdint.h>	arithmetic that reports overflow as a value	chapters 56 and 96
unreachable	promises that control never comes here	chapter 56
memset_explicit	an erase the optimiser may not remove	chapter 80
memalignment	asks a pointer for its alignment	chapter 77
qualifier-preserving search functions	put in a const, get a const back	chapter 80
free_sized, free_aligned_sized	tell the allocator the size you know	chapter 99
realloc(p, 0) is UB (undefined behavior)	an old grey zone moved outside the contract	chapter 46
several time bases, timespec_getres	a monotonic clock, and asking its resolution	chapter 89
timegm	the UTC counterpart of mktime	chapter 89
mbrtoc8, c8rtomb	the UTF-8 pair <uchar.h> was missing	chapter 85
call_once mandated	no longer optional	chapter 94
POSIX functions folded in (strdup and others)	common practice became standard	chapters 79–80
length modifiers for intN_t	printf and scanf know the fixed-width types	chapter 76
binary integer I/O %b	prints and reads 0b	chapter 53
per-header version test macros	a header states what it has	chapter 19
DECIMAL_DIG obsolescent	the per-type macros take over	chapter 28
_Imaginary_I qualifiers adjusted	a small repair in an optional header	chapter 93
harmonised with IEC 60559	follows the floating-point standard more closely	chapter 54
effective-type rules made precise	the ground of aliasing grew clearer	chapter 40

Table 115.3 — What C23 changed in the library

How to use these tables#

• To skim only what is new — follow the “Where” column of the three tables above and you have gone once around C23.
• To check your compiler keeps up — read chapter 19′s edition table together with __STDC_VERSION__ and the per-header version macros. ★ Being in the standard is not being in the compiler — chapter 97 measures two compilers ignoring [[unsequenced]].
• To move old code over — the last section of chapter 97 sets out an order.