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Introduced in C11, and exposed by stdatomic.h. Use _Atomic as a type specifier (ala volatile or const).

  • Unlike other type specifiers, an atomic form of a base type might have different size/alignment than the base type.
  • structs and unions may be declared atomic, but it is then undefined behavior to access any of their members.
  • Implementations are "encouraged to ensure that representation of C and C++ atomic types is the same."


  • _Static_assert() is evaluated at compile time, huzzah!

Generic dispatch

The new _Genetic keyword allows mapping the type of an expression to a set of expressions. This is mainly usable for type-based dispatch of the kind seen in tgmath.h. You can't, so far as I can tell, use it for true parametric polymorphism. Tony Finch demonstrates a clever use for parametric constness:

   #define strchr(s,c) _Generic((s),                    \
       const char * : (const char *)(strchr)((s), (c)), \
       char *       :               (strchr)((s), (c)))




An excellent addition in C99. Type constructions of the form u?int{variety}_t (u prefix denotes unsigned). Varieties include:

  • ptr: Size sufficient to hold a pointer. This is useful for function type definitions, when it's unsure whether a pointer or integer type would be most appropriate for various instances (a void *'s size is not directly related to an int's size by the ANSI C standard).
  • max: Size sufficient to hold any other integer type.
  • fast{8,16,32,64}: Fastest integer type having at least the specified width.
  • least{8,16,32,64}: Minimum native integer type having at least the specified width.
  • {8,16,32,64}: Integer type having precisely this width.


ugly printf specifiers for the types in stdint.h. e.g.:

  • printf("%"PRIu64"\n", some_uint64_tvar);


static in array parameters

Beginning with C99, static can be inserted into array parameters ala:

void foo(int bararr[static 10]);

This indicates to the compiler that the array passed as bararr must have room for at least 10 elements. This ought help it produce diagnostics, and perhaps help code generation.