C17 is a “bugfix release” of the C standard. Whereas the intention of the C working group (WG14) has been that this release does not introduce normative changes (but one), it brings a lot of clarifications all over the place. By adopting this version, some features as implemented by some compilers may change if their interpretation of C11 was different because of an unfortunate ambiguity.
C17 will be superseded by C2x, for which the process of inclusion has begun on the October 2018 of WG14. In particular, a working draft of C2x is now available that is still pretty close to C17:
C2x working draft, post October 2018 meeting
The schedule for C17 was as follows:
- Nov. 2017, adoption by WG14, subject to some minor, editorial changes
- Dec. 2017, integration of these changes and approval by an editorial committee
- Jan. to Mar. 2018, editorial back and forth with ISO, more editorial changes due to new requirements by ISO and their strict enforcement
- Apr 2018, ISO sends out the FDIS (final draft international standard) to the national bodies.
- Apr to June 2018, ballot
- June or July 2018, publication, see C17
I identified the following list of changes in C17 compared to C11. The whole process of clarifications that have been integrated is transparently documented in what we called “defect reports”. So if you urgently need to know about some of these you should look them up, there.
My intention is to write a post on most of the items to explain my POV of what happened. In particular, I will try to cite the new versions of the changed text for reference. Because of copyright issues, I will only be able to do that once C17 has been published by ISO. So please be patient and stay tuned.
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- atomics: initialization (7.17.2, 7.31.8), coherence requirements (5.1.2.4), fences (7.17.3 p11), functions or macros (7.17.1 p6), lockfree depends on type (7.17.5 p3), compare exchange compares memory and not value (7.17.7.3),
atomic_flag(7.17.18) - threads: spurious failures (7.26.3.5 p2, 7.26.3.6 p2, 7.26.4.5 p3), synchronization (7.26.4 p1), thread specific storage (
tss_t) and thread exit (7.26.5 p3, 7.26.5.5, 7.26.6) _Generic(6.5.1.1), rvalues and qualification, cast (6.5.4)- alignment: fundamental alignment (6.2.8),
_Alignas(6.7.5),aligned_alloc(7.22.3.1) - sequence points: full expressions (6.8 p4), full declarators (6.7..6 p3)
- infinite loops (6.8.5 p6)
- reserved identifiers (7.1.3)
- domain or range errors (7.12.1),
ilogb,erfc,lgamma, - underspecification of
clock(7.27.2.1) - underspecification of
reallocfor size 0 (7.22.3.5) - Annex F:
FLT_ROUNDS(F.2 p1) - Annex K:
tmpnam_s,snprintf_s,sprintf_s,vsprintf_s,get_s,mbstowcs_s,wcstombs_s,snwprintf_s,swprintf_s,vsnwprintf_s,vswprint_s,mbsrtowcs_s,wcsrtombs_s
- atomics: initialization (7.17.2, 7.31.8), coherence requirements (5.1.2.4), fences (7.17.3 p11), functions or macros (7.17.1 p6), lockfree depends on type (7.17.5 p3), compare exchange compares memory and not value (7.17.7.3),
NB: comments are switched off for this post. Please communicate errors or imprecisions that you spotted to me directly. If on the other hand you want to discuss the future (or not) of C, there are a lot of places out there. The best that I know of is WG14 itself. So if you really care, please sign in on your committee of your national standards body for programming languages or alike, and invest yourself in the process.
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