c - char[][]和char**的区别，为什么puts(strings[0])不能打印(strings被定义为char**strings)

Question

如何通过调用如下函数来打印字符串数组:

void function(char**string);    //sample1
void function(char string[][LEN2], int size);    //sample2
void function(char (*string)[LEN2], int size);    //sample3

我认为2和3是正确的；

这个问题的正确格式并不重要。

我想知道，计算机如何理解sample1，(尤其是在内存中)而不仅仅是记录正确的答案。

谢谢。(我第一次使用，可能有点可笑。)

使用Visual Studio 2017，关闭安全检查。在PC上运行。

#include<stdio.h>
#define LEN1 10
#define LEN2 100
void item1(char**string);
void print_initial_string(char**string);

int main(void)
{
    char string[LEN1][LEN2] = {"a", "ab", "abc", "abcd", "abcde",
            "c", "cd", "cde", "cdgh", "seids"};
    item1(string);
}

/*implements of functions*/
void item1(char**string)
{
    print_initial_string(string);
}
void print_initial_string(char**string)
{
    char (*c)[LEN2] = string[0];
    for (int i = 0; i < LEN1; i++)
        puts(c);     /*-- stopped at here --*/
}

我认为它会打印字符串，但失败了。

并返回代码-1073741819

Answer 1

如果我编译做 gcc -pedantic -Wextra ar.c我收到很多表明问题的消息:

pi@raspberrypi:~/Downloads $ gcc -pedantic -Wextra ar.c
ar.c: In function ‘main’:
ar.c:11:11: warning: passing argument 1 of ‘item1’ from incompatible pointer type [-Wincompatible-pointer-types]
     item1(string);
           ^~~~~~
ar.c:4:6: note: expected ‘char **’ but argument is of type ‘char (*)[100]’
 void item1(char**string);
      ^~~~~
ar.c: In function ‘print_initial_string’:
ar.c:21:23: warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]
     char (*c)[LEN2] = string[0];
                       ^~~~~~
ar.c:23:14: warning: passing argument 1 of ‘puts’ from incompatible pointer type [-Wincompatible-pointer-types]
         puts(c);     /*-- stopped at here --*/
              ^
In file included from ar.c:1:0:
/usr/include/stdio.h:697:12: note: expected ‘const char *’ but argument is of type ‘char (*)[100]’
 extern int puts (const char *__s);
            ^~~~

<小时/>

char**string是 char* 的数组，这意味着 string 中的每个条目都是 char* (所以是一个指针)，这不是 char string[LEN1][LEN2] 因为它不包含指针

所以void item1(char (*string)[LEN2])和void print_initial_string(char (*string)[LEN2])

char (*c)[LEN2] = string[0];也不好，c是一个char*，但你说它是一个指向 char[LEN2] 的指针，你想要char (*c)[LEN2] = &string[0];或者只是 char (*c)[LEN2] = string; 。在这种情况下puts(c)必须是puts(c[i]);因为c不是一个字符串而是一个指向

的指针

最后:

#include<stdio.h>

#define LEN1 10
#define LEN2 100

void item1(char (*string)[LEN2]);
void print_initial_string(char (*string)[LEN2]);

int main(void)
{
    char string[LEN1][LEN2] = {"a", "ab", "abc", "abcd", "abcde",
                               "c", "cd", "cde", "cdgh", "seids"};
    item1(string);
}

/*implements of functions*/
void item1(char (*string)[LEN2])
{
    print_initial_string(string);
}
void print_initial_string(char (*string)[LEN2])
{
  char (*c)[LEN2] = string;

  for (int i = 0; i < LEN1; i++)
    puts(c[i]);
}

编译和执行:

pi@raspberrypi:~/Downloads $ gcc -g -pedantic -Wextra ar.c
pi@raspberrypi:~/Downloads $ ./a.out
a
ab
abc
abcd
abcde
c
cd
cde
cdgh
seids

在valgrind下执行:

pi@raspberrypi:~/Downloads $ valgrind ./a.out
==11987== Memcheck, a memory error detector
==11987== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==11987== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==11987== Command: ./a.out
==11987== 
a
ab
abc
abcd
abcde
c
cd
cde
cdgh
seids
==11987== 
==11987== HEAP SUMMARY:
==11987==     in use at exit: 0 bytes in 0 blocks
==11987==   total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated
==11987== 
==11987== All heap blocks were freed -- no leaks are possible
==11987== 
==11987== For counts of detected and suppressed errors, rerun with: -v
==11987== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)

<小时/>

一个示例，其中 char**是正确的:

#include <stdio.h>
#include <stdlib.h>

#define LEN 10

void pr(const char ** a, int sz)
{
  for (int i = 0; i != sz; ++i)
    puts(a[i]);
}

int main()
{
  const char *a[LEN] = { "a", "ab", "abc", "abcd", "abcde",
                         "c", "cd", "cde", "cdgh", "seids"};

  pr(a, LEN);
}

我使用const，因为文字字符串是const

编译和执行:

pi@raspberrypi:/tmp $ gcc -pedantic -Wall aa.c
pi@raspberrypi:/tmp $ ./a.out
a
ab
abc
abcd
abcde
c
cd
cde
cdgh
seids