理解 C 中的解引用、取址和数组下标运算符 [英] Understanding the dereference, address-of, and array subscript operators in C

问题描述

我将 argv[] 定义为 char *.使用以下 printf 语句:

I have argv[] defined as a char *. Using the following printf statements:

     printf("%s\n",argv[1]);   // prints out the entire string
     printf("%p\n",&argv[1]);  // & -> gets the address
     printf("%c\n",argv[1][0]);// prints out the first char of second var
     printf("%c\n",*argv[1]);  //  

这是最后一个我不明白.打印 *argv[1] 是什么意思?为什么和 *argv[1][0] 不一样，为什么不能打印出 printf("%s\n",*argv[1]);.另外，为什么 &*argv[1] 和 &argv[1] 是不同的地址?

It's this last one I don't understand. What does it mean to print *argv[1]? why isn't that the same as *argv[1][0] and how come you can't print out printf("%s\n",*argv[1]);. Also, why is &*argv[1] a different address then &argv[1]?

推荐答案

数组下标操作 a[i] 定义为 *(a + i) - given地址 a，从该地址偏移 i 元素(不是字节)并取消引用结果.因此，给定一个指针p，*p等价于*(p + 0)，等价于p[0].

The array subscript operation a[i] is defined as *(a + i) - given the address a, offset i elements (not bytes) from that address and dereference the result. Thus, given a pointer p, *p is equivalent to *(p + 0), which is equivalent to p[0].

argv 的类型为 char **;鉴于此，以下所有情况都是正确的:

The type of argv is char **; given that, all of the following are true:

    Expression         Type            Value
    ----------         ----            -----
          argv         char **         Pointer to a sequence of strings
         *argv         char *          Equivalent to argv[0]
        **argv         char            Equivalent to argv[0][0]
       argv[i]         char *          Pointer to a single string
      *argv[i]         char            Same as argv[i][0]
    argv[i][j]         char            j'th character of i'th string
      &argv[i]         char **         Address of the pointer to the i'th string

由于argv[i][j]的类型是char，*argv[i][j]是无效的表达.

Since the type of argv[i][j] is char, *argv[i][j] is not a valid expression.

这是 argv 序列的一个糟糕的可视化:

Here's a bad visualization of the argv sequence:

     +---+              +---+                                         +---+
argv |   | ---> argv[0] |   | ---------------------------> argv[0][0] |   |
     +---+              +---+                     +---+               +---+
                argv[1] |   | -------> argv[1][0] |   |    argv[0][1] |   |
                        +---+                     +---+               +---+
                         ...           argv[1][1] |   |                ...
                        +---+                     +---+               +---+
             argv[argc] |   | ---|||               ...   argv[0][n-1] |   |
                        +---+                     +---+               +---+
                                     argv[1][m-1] |   |
                                                  +---+

这可能有助于解释不同表达式的结果.

This may help explain the results of different expressions.

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