c strcpy

zj@zj:~/C_parm/string/own_str/strcpy$ cat strcpy.c
/*
  下面是strcpy库函数的实现，因为库函数讲究的就是精练、简洁。所以没有其他的异常处理代码。主要的异常处理还是交给了函数的使用者，在调用前请确认目的和源指针是否都存在（不能为Null），请确认目标指针空间是否大于源字符串的空间。
   Copies the string src into the spot specified by dest; assumes enough room.
目标指针空间必须大于源字符串空间。
*/
#include<stdio.h>
#include<stdlib.h>

#define MAX_LEN 255
char* my_strcpy(char* dst, const char* src);

int main()
{
   char a[MAX_LEN];
   char* str = "Hello,world!";
   printf("Copied:%s\n",my_strcpy(a,str));
   exit(EXIT_SUCCESS);
}

char* my_strcpy(char* dst, const char* src)
{
    if((dst==NULL)||(src==NULL))
          perror("dst or src is null");
    char* cp = dst;
    while((*cp++=*src++)!='\0')
         ;
    return (dst);
}

zj@zj:~/C_parm/string/own_str/strcpy$ ./strcpy
Copied:Hello,world!



zj@zj:~/C_parm/string/own_str/strncpy$ cat strncpy.c
/*Copies count characters from the source string to the destination.
 * If count is less than the length of source,NO NULL CHARACTER is put
 * onto the end of the copied string.If count is greater than the length
 *of sources, dest is padded with null characters to length count.
 *
 * 把src所指由NULL结束的字符串的前n个字节复制到dest所指的数组中.如果src的
 * 前n个字节不含NULL字符，则结果不会以NULL字符结束;
 *如果src的长度小于n个字节，则以NULL填充dest直到复制完n个字节。
 *src和dest所指内存区域不可以重叠且dest必须有足够的空间来容纳src的字符串
 *返回指向dest的指针.
*/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

#define MAX_LEN 255
char* my_strncpy( char* dest, const char* source, int count );

int main()
{
   char dst1[MAX_LEN];
   char dst2[MAX_LEN];
   char* src = "Hello,world!";
   bzero(dst1,'\0');
   bzero(dst2,'\0');
   printf("After strncpy:%s\n",my_strncpy(dst1,src,15));
   printf("After strncpy:%s\n",my_strncpy(dst2,src,10));
   
   exit(EXIT_SUCCESS);
}

char* my_strncpy( char* dest, const char* source, int count )
{
  if((dest==NULL)||(source==NULL))
    perror("dest or source null");
  char *p = dest;
  while (count && (*dest++ = *source++)) count--;
  while(count--)
       *dest++ = '\0';
  /*由于dest已经移动,返回的时候返回首地址就可以了*/
    return(p);
 
  /*while (count && (*p++ = *source++)) count--;
   while(count--)
       *p++ = '\0';
   return(dest);*/
}

zj@zj:~/C_parm/string/own_str/strncpy$ ./strncpy
After strncpy:Hello,world!
After strncpy:Hello,worl ???????