typeof在linux中妙用

typeof在linux中妙用

今天偶然又看到了typeof这个东西，只知道它大概是返回变量的类型，后来上网查了下发现这个关键字在linux中用的非常多， 这里大概叙述了typeof是一个什么东西，怎么用，实际上可以用简单的话来重述。如果你对sizeof很熟悉的话，那么大可进行类推，sizeof（exp.）返回的是exp.的数据类型大小，那么typeof（exp.）返回的就是exp.的数据类型。值得注意的是在上面的话里我们可以看到，如果编译选项中指定了使用标准C，那么gcc的扩展使用可能会受到影响，不过在gcc编译条件下使用__typeof__依然可以正常工作，这和使用asm是一样的。当然如果是在其他的编译器条件下，这样做也不行了，只能自定义一个macro去使用，也就是说跟gcc没啥关系了，你愿意把typeof咋实现都可以。

下面写一个小程序示例一下:)

#include <stdio.h>

typedef struct 
{
 int x;
 char y;
}astruct, * pastrcut;

int main()
{
 int sizem, sizew;
 int x = 3;
 typeof(&x) m;
 sizem = sizeof(m);
 *m = 5;
 typeof(((astruct *)5)->y) w;
 sizew = sizeof(w);
 w = ''a'';
 return 1;
}

首先看main函数里的m变量，这个变量的类型就是typeof(&x)， 由于x是int型的（这里与x是否被赋值一点关系都没有）所以&x应该是int *类型，那么typeof(&x)返回的类型就是int*，所以m自然也就是个int*类型的。之后我们看w变量，其类型是 typeof(((astruct *)8)->y)， 其中astruct是一个被定义的结构类型，其中的y元素是char*类型，那么((astruct *)8)->y是啥意思呢？在这里0并不是真正的变量，可以把它理解为一个替代使用的符号当然这个符号最好是一个数，其意思更可以理解为一个被赋值了的变量，这个数可以不是0，3也可以8也可以，随便什么都可以。那么((astruct *)0)->y仅仅就是表示了y这个变量，所以typeof的结果就是y元素的类型，也就是char。

gcc：

6.6 Referring to a Type with typeof

Another way to refer to the type of an expression is with typeof. The syntax of using of this keyword looks like sizeof, but the construct acts semantically like a type name defined with typedef.

There are two ways of writing the argument to typeof: with an expression or with a type. Here is an example with an expression:

     typeof (x[0](1))

This assumes that x is an array of pointers to functions; the type described is that of the values of the functions.

Here is an example with a typename as the argument:

     typeof (int *)

Here the type described is that of pointers to int.

If you are writing a header file that must work when included in ISO C programs, write __typeof__ instead of typeof. See Alternate Keywords.

A typeof construct can be used anywhere a typedef name can be used. For example, you can use it in a declaration, in a cast, or inside of sizeof or typeof.

The operand of typeof is evaluated for its side effects if and only if it is an expression of variably modified type or the name of such a type.

typeof is often useful in conjunction with statement expressions (see Statement Exprs). Here is how the two together can be used to define a safe “maximum” macro which operates on any arithmetic type and evaluates each of its arguments exactly once:

     #define max(a,b) \
       ({ typeof (a) _a = (a); \
           typeof (b) _b = (b); \
         _a > _b ? _a : _b; })

The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts.

Some more examples of the use of typeof:

• This declares y with the type of what x points to.

            typeof (*x) y;
       

• This declares y as an array of such values.

            typeof (*x) y[4];
       

• This declares y as an array of pointers to characters:

            typeof (typeof (char *)[4]) y;
       

  It is equivalent to the following traditional C declaration:

            char *y[4];
       

  To see the meaning of the declaration using typeof, and why it might be a useful way to write, rewrite it with these macros:

            #define pointer(T)  typeof(T *)
            #define array(T, N) typeof(T [N])
       

  Now the declaration can be rewritten this way:

            array (pointer (char), 4) y;
       

  Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char.

Compatibility Note: In addition to typeof, GCC 2 supported a more limited extension that permitted one to write

     typedef T = expr;

with the effect of declaring T to have the type of the expression expr. This extension does not work with GCC 3 (versions between 3.0 and 3.2 crash; 3.2.1 and later give an error). Code that relies on it should be rewritten to use typeof:

     typedef typeof(expr) T;

This works with all versions of GCC.