C 语言中一些非常用知识点 备忘录

在看一些C语言资料时，总是碰到一些非常见的知识点，看过就忘，这里把他们记录下来。

7. ----> 什么是Designated initializers for aggregate types (C only),看了IBM的解释更易懂，

Designated initializers, a C99 feature, aresupported for aggregate types, including arrays, structures, and unions. Adesignated initializer, ordesignator, points outa particular element to be initialized. Adesignatorlist is a comma-separated list of one or more designators. A designatorlist followed by an equal sign constitutes adesignation.

• Elements within an aggregate can be initialized in any order.
• The initializer list can omit elements that are declared anywhere in theaggregate, rather than only at the end.Elements that areomitted are initialized as if they are static objects: arithmetic types areinitialized to 0; pointers are initialized to NULL.
• Where inconsistent or incomplete bracketing of initializers for multi-dimensionalarrays or nested aggregates may be difficult to understand, designators canmore clearly identify the element or member to be initialized.

下面举例：

struct xyz {       int a;       int b;       int c;       } klm = { .a = 99, .c = 100 };

int aa[4] = { [2] = 3, [1] = 6 };
static short grid[3] [4] = { [0][0]=8,[0][3]=1,                             [2][0]=9, [2][1]=3,                             [2][2]=1, [2][3]=1 };

6.---->C99 permits declarations anywhere within a block. Previous Versions of C permitted them only before the first statment. 参考（C语言参考手册） chapter 4.3

5.----> Statement labels have function scope. 意思就是说用于goto 语句的标号的作用范围是一个函数内。-reference 《C A reference manual》chapter:4.2.1;8.10

4.---->什么是 incomplete type?

  如果有如下定义：

  structure foo;

  structure foo *pFoo;

then the expression  *pFoo is an experession with incomplete type,because the type srtucture foo have not been fully defined.

3.----> 什么是 qualified type,什么是 fully qualified type?

2.---->The C99 function specifier(inline) can appear only on function declarations. -----reference 《C A reference manual》（C语言参考手册） chapter 4.1

1.---->声明一个东西时（一个变量，一个类型，一个函数等），我们需要declaration-specifiers,总共有如下四种specifiers: 参考（C语言参考手册） chapter 4

storage-class-specifier,   type-specifier,    type-qualifier,   function-specifier

其中 storage-class-specifier 有：auto, extern,register,static,typedef

其中Type specififiers 主要提供数据类型信息，有 enumeration，floating-point,interger,structure,typedef-name,union,void

其中type qualitifer 主要是提供更多额外的信息，尤其是在accessing objects of the type through lvalues.  有：const,volatile,restrict.