41.内存函数实现(memcpy,memset,memmove,memicmp,memchr.memccpy)

• memcpy

  #include <stdio.h>
  #include <stdlib.h>
  #include <memory.h>
  
  void * mymemcpy(void* _Dst, void const* _Src, size_t _Size)
  {
      if (_Dst == NULL || _Src == NULL)
      {
          return NULL;
      }
      
      //转化成char类型,一次前进一个字节
      char *dst = _Dst;
      char *src = _Src;
  
      for (int i = 0; i < _Size; i++)
      {
          dst[i] = src[i];
      }
  
      return _Dst;
  }
  
  void main()
  {
      int a[10] = { 1,2,3,4,5,6,7,8,9,10 };
      int *p = malloc(sizeof(int) * 10);
      int *tmp = mymemcpy(p, a, 40);
  
      for (int i = 0; i < 10; i++)
      {
          printf("%d
  " ,p[i]);
      }
  
      char str[100] = "hello world";
      char mystr[100];
      mymemcpy(mystr, str, strlen(str) + 1);
  
      printf("%s
  ", mystr);
  
      system("pause");
  }

• memset

  void *mymemset(void *_Dst, int _Val, size_t _Size)
  {
      if (_Dst == NULL || _Val == NULL)
      {
          return;
      }
  
      for (int i = 0; i < _Size; i++)
      {
          ((char *)_Dst)[i] = _Val;
      }
  
      return _Dst;
  }

• memmove

  void * mymemmove(void *_Dst, const void *_Src, size_t _Size)
  {
      if (_Dst == NULL || _Src == NULL)
      {
          return NULL;
      }
  
      void *psrc = malloc(_Size);//分配内存
      memcpy(psrc, _Src, _Size);
      memcpy(_Dst, psrc, _Size);
      free(psrc);
      return _Dst;
  }

• memicmp(比较指定字符串前n个字符串)

  int mymemicmp(const void *_Buf1, const void *_Buf2, size_t _Size)
  {
      //保存变量
      char *buf1 = _Buf1;
      char *buf2 = _Buf2;
  
      //结束标识
      char *end = buf1 + _Size;
  
      while ((*buf1 == *buf2) && buf1 != end)
      {
          buf1++;
          buf2++;
      }
  
      if (buf1 == end)
      {
          return 0;
      }
      else
      {
          return *buf1 - *buf2 > 0 ? 1 : -1;
      }
  }

• memchr(寻找一个字符串中是否有指定字符)

  void *mymemchr(void *start, char ch, int maxlength)
  {
      char *p = NULL;
      for (int i = 0; i < maxlength; i++)
      {
          if (((char *)start)[i] == ch)
          {
              p = (char *)start + i;
              break;
          }
      }
      return p;
  }

• memccpy(复制n个字符,遇到指定值退出)

  void * mymemccpy(void * _Dst, const void *_Src, int _Val,size_t _MaxCount)
  {
      char *dst = _Dst;
      char *src = _Src;
  
      for (int i = 0; i < _MaxCount; i++)
      {
          dst[i] = src[i];
          if (dst[i] == _Val)
          {
              break;
          }
      }
  }