memcpy strcpy strncpy

/* Public domain.  */
#include<stddef.h>


void*
memcpy (void*dest,constvoid*src, size_t len)
{
  char*d= dest;
  constchar*s= src;
  while(len--)
    *d++=*s++;
  return dest;
}
//参数没有类型之分   是以字节为单位，放回第一个参数的指针  
 
 
#include <stdio.h>
#include <string.h>
int main ()
{
   const char src[50] = "http://www.tutorialspoint.com";
   char dest[50];
   printf("Before memcpy dest = %s\n", dest);
   memcpy(dest, src, strlen(src)+1);
   printf("After memcpy dest = %s\n", dest);
   
   return(0);
}
 
Before memcpy dest =
After memcpy dest = http://www.tutorialspoint.com
 
 
char* strcpy(char*to,constchar*from);


char* strcpy(char*to,constchar*from)
{
  char*save= to;


  for(;(*to=*from);++from,++to);
  return save;
}
//以/0为结束
#include <stdio.h>
#include <string.h>
int main()
{
   char src[40];
   char dest[100];
  
   memset(dest, '\0', sizeof(dest));
   strcpy(src, "This is tutorialspoint.com");
   strcpy(dest, src);
   printf("Final copied string : %s\n", dest);
   
   return(0);
}
Let us compile and run the above program, this will produce the following result:
Final copied string : This is tutorialspoint.com
 
 
#include "LibC.h" 
char*
 strncpy(char* dest, const char*
 src, size_t n) 
{ 
 
  char * ptr =
 dest; 
 
  while (n && (*ptr = *src)) 
 
  { 
 
     ++ptr; 
 
     ++src; 
 
     --n; 
 
  } 
 
  while (n--) 
 
     *ptr++ = '\0'; 
 
  return dest; 
} 
//尽量让n大这样可以防止dest中没有/0
#include <stdio.h>
#include <string.h>
int main()
{
   char src[40];
   char dest[12];
  
   memset(dest, '\0', sizeof(dest));
   strcpy(src, "This is tutorialspoint.com");
   strncpy(dest, src, 10);
   printf("Final copied string : %s\n", dest);
   
   return(0);
}
Final copied string : This is tu