/* strcpy: 字符串拷贝 */
#include <stdio.h>
#include <string.h>
int main()
{
  char arr1[20] = "xxxxxxxxxx";
  char arr2[] = "hello";
  strcpy(arr1, arr2);   // 字符串拷贝(目标字符串，源字符串)
  printf("%s\n", arr1); // hello
  return 0;
}

#include <stdio.h>
char* my_strcpy (
  char* dest, // 目标字符串
  char* src   // 源字符串
  )
{
  while (*src != '\0') {
  *dest = *src;
  dest++;
  src++;
  }
  *dest = *src; // 拷贝'\0'
}
int main()
{
  char arr1[20] = "xxxxxxxxxx";
  char arr2[] = "hello";
  my_strcpy(arr1, arr2);
  printf("%s\n", arr1); // hello
  return 0;
}

#include <stdio.h>
char* my_strcpy (char* dest, char* src)
{
  while (*src != '\0') {
  *dest++ = *src++;
  }
  *dest = *src;
}

char* my_strcpy (char* dest, char* src)
{
  while (*dest++ = *src++) // 既拷贝了斜杠0，又使得循环停止
  ;
}

#include <stdio.h>
#include <assert.h>
char* my_strcpy(char* dest, char* src)
{
  assert(dest != NULL); // 断言   "dest不能等于NULL"
  assert(src != NULL);  // 断言   "src 不能等于NULL"
  while (*dest++ = *src++)
  ;
}
int main()
{
  char arr1[20] = "xxxxxxxxxx";
  char arr2[] = "hello";
  my_strcpy(arr1, NULL); // 👈 实验：传入一个NULL
  printf("%s\n", arr1);
  return 0;
}

int main()
{
  int num = 10;
  int* p = #
  *p = 20;
  printf("%d\n", num);
  return 0;
}

int main()
{
    const int num = 10;
    int* p = #
    *p = 20;
    printf("%d\n", num);
    return 0;
}

int main()
{
    const int num = 10;
    const int* p = # 
// 如果放在 * 左边，修饰的是 *p，表示指针指向的内容，是不能通过指针来改变的
    *p = 20; // ❌ 不可修改
    printf("%d\n", num);
    return 0;
}

int main()
{
    const int num = 10;
    int n = 100;
    const int* p = #
    *p = 20 // ❌ 不能修改
    p = &n; // ✅ 但是指针变量的本身是可以修改的
    printf("%d\n", num);
    return 0;
}

int main()
{
    const int num = 10;
    int n = 100;
    int* const  p = #
// 如果放在 * 右边，修饰的是指针变量p，表示的指针变量不能被改变
// 但是指针指向的内容，可以被改变
    p = 20; // ✅ 可以修改
    p = &n; // ❌ 不能修改
    printf("%d\n", num);
    return 0;
}

int main()
{
    const int num = 10;
    const int* const p = #
    int n = 100;
    *p = 20; // ❌ 不能修改
    p = &n;  // ❌ 不能修改
    printf("%d\n", num);
    return 0;
}

char* my_strcpy (char* dest, const char* src)
{
  assert(dest != NULL);
  assert(src != NULL);
  // while(*src++ = *dest) 👈 防止写反，加一个const
  while (*dest++ = *src++)
  ;
}

#include <stdio.h>
#include <assert.h>
char* my_strcpy (char* dest,const char* src)
{
  char* ret = dest; // 在刚开始的时候记录一下dest
  assert(dest != NULL);
  assert(src != NULL);
  while (*dest++ = *src++)
  ;
  return ret; // 最后返回dest
}
int main()
{
  char arr1[20] = "xxxxxxxxxx";
  char arr2[] = "hello";
  printf("%s\n", my_strcpy(arr1, arr2)); // 链式访问
  return 0;
}

/***
*char *strcpy(dst, src) - copy one string over another
*
*Purpose:
*       Copies the string src into the spot specified by
*       dest; assumes enough room.
*
*Entry:
*       char * dst - string over which "src" is to be copied
*       const char * src - string to be copied over "dst"
*
*Exit:
*       The address of "dst"
*
*Exceptions:
*******************************************************************************/
char * strcpy(char * dst, const char * src)
{
        char * cp = dst;
        assert(dst && src);
        while( *cp++ = *src++ )
               ;     /* Copy src over dst */
        return( dst );
}

#include <stdio.h>
#include <assert.h>
int my_strlen(const char* str)
{
  assert(str);
  int count = 0;
  while (*str) {
  count++;
  str++;
  }
  return count;
}
int main()
{
  char arr[] = "abcdef";
        int len = my_strlen(arr);
  printf("%d\n", len);
  return 0;
}

#include <stdio.h>
#include <assert.h>
size_t my_strlen(const char* str)
{
  assert(str);
  const char* eos = str;
  while (*eos++);
  return(eos - str - 1);
}
int main()
{
  char arr[] = "abcdef";
  printf("%d\n", my_strlen(arr));
  return 0;
}

/***
*strlen.c - contains strlen() routine
*
*       Copyright (c) Microsoft Corporation. All rights reserved.
*
*Purpose:
*       strlen returns the length of a null-terminated string,
*       not including the null byte itself.
*
*******************************************************************************/
#include <cruntime.h>
#include <string.h>
#pragma function(strlen)
/***
*strlen - return the length of a null-terminated string
*
*Purpose:
*       Finds the length in bytes of the given string, not including
*       the final null character.
*
*Entry:
*       const char * str - string whose length is to be computed
*
*Exit:
*       length of the string "str", exclusive of the final null byte
*
*Exceptions:
*
*******************************************************************************/
size_t __cdecl strlen (
        const char * str
        )
{
        const char *eos = str;
        while( *eos++ ) ;
        return( eos - str - 1 );
}
size_t ：无符号整型（unsigned int）
__cdecl ：函数调用约定