difference between c99 and c11

Per the C 2011 standard itself, here are the major changes from C99:

Foreword
...
6     This third edition cancels and replaces the second edition, ISO/IEC 9899:1999, as corrected by ISO/IEC 9899:1999/Cor 1:2001, ISO/IEC 9899:1999/Cor 2:2004, and ISO/IEC 9899:1999/Cor 3:2007. Major changes from the previous edition include:

     — conditional (optional) features (including some that were previously mandatory)

     — support for multiple threads of execution including an improved memory sequencing model, atomic objects, and thread-local storage (<stdatomic.h> and <threads.h>)

     — additional floating-point characteristic macros (<float.h>)

     — querying and specifying alignment of objects (<stdalign.h>, <stdlib.h>)

     — Unicode characters and strings (<uchar.h>) (originally specified in ISO/IEC TR 19769:2004)

     — type-generic expressions

     — static assertions

     — anonymous structures and unions

     — no-return functions

     — macros to create complex numbers (<complex.h>)

     — support for opening files for exclusive access

     — removed the gets function (<stdio.h>)

     — added the aligned_alloc, at_quick_exit, and quick_exit functions (<stdlib.h>)

     — (conditional) support for bounds-checking interfaces (originally specified in ISO/IEC TR 24731−1:2007)

     — (conditional) support for analyzability

Good overviews of C11 standard:

• https://en.wikipedia.org/wiki/C11_(C_standard_revision)
• http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
• https://smartbear.com/blog/test-and-monitor/c11-a-new-c-standard-aiming-at-safer-programming/

The standard includes several changes to the C99 language and library specifications, such as:

• Alignment specification (_Alignas specifier, _Alignof operator, aligned_alloc function, <stdalign.h> header file)
• The _Noreturn function specifier and the <stdnoreturn.h> header file

• Type-generic expressions using the _Generic keyword. For example, the following macro cbrt(x) translates to cbrtl(x), cbrt(x) or cbrtf(x) depending on the type of x:

  #define cbrt(x) _Generic((x), long double: cbrtl, \
                            default: cbrt, \
                            float: cbrtf)(x)
  

• Multi-threading support (_Thread_local storage-class specifier, <threads.h> header including thread creation/management functions, mutex, condition variable and thread-specific storage functionality, as well as the _Atomic type qualifier and <stdatomic.h> for uninterruptible object access).

• Improved Unicode support based on the C Unicode Technical Report ISO/IEC TR 19769:2004 (char16_t and char32_t types for storing UTF-16/UTF-32 encoded data, including conversion functions in <uchar.h> and the corresponding u and U string literal prefixes, as well as the u8 prefix for UTF-8 encoded literals).
• Removal of the gets function, deprecated in the previous C language standard revision, ISO/IEC 9899:1999/Cor.3:2007(E), in favor of a new safe alternative, gets_s.
• Bounds-checking interfaces (Annex K).
• Analyzability features (Annex L).
• More macros for querying the characteristics of floating point types, concerning subnormal floating point numbers and the number of decimal digits the type is able to store.
• Anonymous structures and unions, useful when unions and structures are nested, e.g. in struct T { int tag; union { float x; int n; }; };.
• Static assertions, which are evaluated during translation at a later phase than #if and #error, when types are understood by the translator.
• An exclusive create-and-open mode ("…x" suffix) for open. This behaves like O_CREAT|O_EXCL in POSIX, which is commonly used for lock files.
• The quick_exit function as a third way to terminate a program, intended to do at least minimal deinitialization if termination with exit fails.
• Macros for the construction of complex values (partly because real + imaginary*I might not yield the expected value if imaginary is infinite or NaN).