Q:

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

As I was checking news about GCC 8, I saw that they added support for the 2017 version of the C language (not C++17, really C17). But I can't find any information about it on Internet.
Is it a new ISO version like C11, or just a codename used by the GCC team for some corrections in their compiler ?

A:

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]

A:

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.

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.

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.

