字符串复制函数：

strcpy(目标字符串的首地址,被复制的字符串的首地址)

  复制p到a：

	char * p="12345";char a[20];strcpy(a, p);printf("%s\n", a);

 strcpy_s(目标字符串的首地址,缓冲长度,被复制的字符串的首地址)

	strcpy_s(a, 20, p);//20为缓冲长度 可用空间的长度printf("%s\n", a);

strncpy(目标字符串的首地址,被复制的字符串的首地址,复制的长度) ，这种复制方式更像是覆盖

	strncpy(a, "123456", 4);

strncpy_s(目标字符串的首地址,缓冲长度,被复制的字符串的首地址,,复制的长度)，清零后复制

	strncpy_s(a, 20, "123456", 4);

仿真字符串复制函数 strcpy

char* my_strcpy(char* dest, const char* src);char* my_strcpy(char* dest, const char* src)
{char* p_dest = dest;const char* p_src = src;while (*p_src)//!='\0'{*p_dest = *p_src;p_src++;p_dest++;}*p_dest = '\0';return dest;
}

测试：

printf("%s\n", my_strcpy(a, "how are you"));

仿真字符串复制函数 strncpy

char* my_strncpy(char* dest, const char* src, unsigned num);char* my_strncpy(char* dest, const char* src, unsigned num)
{char* p_dest = dest;const char* p_src = src;int i = 0;while (*p_src && i++ < num)//!='\0'{*p_dest = *p_src;p_src++;p_dest++;}//*p_dest = '\0';return dest;
}

测试：

  my_strncpy(a, "123456", 4);

完整代码：

#include <stdio.h>
#include <string.h>
char* my_strcpy(char* dest, const char* src);
char* my_strncpy(char* dest, const char* src, unsigned num);
int main ()
{/*字符串复制函数*/ char * p="12345";char a[20];/*strcpy*///a=p;strcpy(a, p);printf("%s\n", a);printf("%s\n", my_strcpy(a, "how are you"));printf("%d\n", strlen(strcpy(a, "how old are you")));/*strcpy_s*/strcpy_s(a, 20, p);//20为缓冲长度 可用空间的长度printf("%s\n", a);/*strncpy*/my_strncpy(a, "123456", 4);//像是覆盖strncpy(a, "123456", 4);//像是覆盖printf("%s\n", a);/*strncpy_s*/strncpy_s(a, 20, "123456", 4);//清零后复制printf("%s\n", a);/*仿真字符串复制函数 strcpy   */return 0;
}char* my_strcpy(char* dest, const char* src)
{char* p_dest = dest;const char* p_src = src;while (*p_src)//!='\0'{*p_dest = *p_src;p_src++;p_dest++;}*p_dest = '\0';return dest;
}char* my_strncpy(char* dest, const char* src, unsigned num)
{char* p_dest = dest;const char* p_src = src;int i = 0;while (*p_src && i++ < num)//!='\0'{*p_dest = *p_src;p_src++;p_dest++;}//*p_dest = '\0';return dest;
}

完整输出：