What is C17 and what changes have been made to the language?

According to GCC reference, C17 is actually a bug-fix version of the C11 standard with DR resolutions integrated.

C17, a bug-fix version of the C11 standard with DR [Defect Report] resolutions integrated, will soon go to ballot. This patch adds corresponding options -std=c17, -std=gnu17 (new default version, replacing -std=gnu11 as the default), -std=iso9899:2017. As a bug-fix version of the standard, there is no need for flag_isoc17 or any options for compatibility warnings; however, there is a new __STDC_VERSION__ value, so new cpplib languages CLK_GNUC17 and CLK_STDC17 are added to support using that new value with the new options. (If the standard ends up being published in 2018 and being known as C18, option aliases can be added. Note however that -std=iso9899:199409 corresponds to a __STDC_VERSION__ value rather than a publication date.)

(There are a couple of DR resolutions needing implementing in GCC, but that's independent of the new options.)

So, there are no new features included in C17.

The Cppreference (History of C) says:

Future development

C17 Next minor C language standard revision, will include all accepted C11 defect reports, but no new features.

UPDATE:

• 2018: C17 (ISO/IEC 9899:2018) (ISO Store) (Final draft) Includes the deprecation of ATOMIC_VAR_INIT and the fixes to the following defect reports:

[DR 400], [DR 401], [DR 402], [DR 403], [DR 404], [DR 405], [DR 406], [DR 407], [DR 410], [DR 412], [DR 414], [DR 415], [DR 416], [DR 417], [DR 419], [DR 423], [DR 426], [DR 428], [DR 429], [DR 430], [DR 431], [DR 433], [DR 434], [DR 436], [DR 437], [DR 438], [DR 439], [DR 441], [DR 444], [DR 445], [DR 447], [DR 448], [DR 450], [DR 452], [DR 453], [DR 457], [DR 458], [DR 459], [DR 460], [DR 462], [DR 464], [DR 465], [DR 468], [DR 470], [DR 471], [DR 472], [DR 473], [DR 475], [DR 477], [DR 480], [DR 481], [DR 485], [DR 487], [DR 491]

C17 is a “bugfix release” of the C standard how "M.S Chaudhari" noticed. However, there is very useful information prepared by Jens Gustedt the author of "Modern C" book.

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He identified the following list of changes in C17 compared to C11:

• 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 realloc for 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.

Link to the main page C17.

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Also, this content will be updated by Jens you can follow to update here Jeans Gustedt Blog.

P.S: before posting all this stuff I received approval from the author.