copy last characters of strings?

Shahid nx · 08-17-2012, 06:30 AM

Hello Everyone,
Need urgent help. From a string how can i copy last some charterers. like by using this function strncpy (str2,str1,5); will copy first 5 character from str1 to str2 But how can i copy last say 10 character from one string to another. And also sicne i m using array of string so please give me solutuion using sprintf().

Thanks ,
shahid nx

pixellany · 08-17-2012, 07:37 AM

NOT urgent---We do things when we get to it. If you are on a tight schedule, you might want to hire someone.

I assume you are using C----is there not a function to get the length of a string? Once you know the length of the string, just count back from there.

tronayne · 08-17-2012, 08:51 AM

There are a bunch of different ways you can skin this cat; possibly the easiest is to simply determine where to start and where to stop.

All strings are arrays, starring at zero and ending at some point with a NULL (\0). So you use the strlen() function to find out how long your incoming string is, subtract 10 from that value then copy character by character to your destination string until you hit the NULL (and be sure to copy the NULL!). Something like this will give the starting point and the length (the stopping point):

Code:

n = strlen (src);       /* how long is src        */
i = n - 10              /* starting point in src  */
j = 0;                  /* starting point in dest */

You need to make sure that i is not less than zero -- even if you believe that your source string is always going to be some length greater than ten you need to cover yourself when it isn't, just in case. So check it

Code:

if (i < 0)
     i = n;

Then all you need to do is

Code:

for ( ; i < n && src [i] != '\0'; i++) {
     dest[j] = src[i];
     j++;
}
dest[j] = '\0';

Here's the whole thing

Code:

#include <stdio.h>
#include <string.h>

void	main	(void)
{
	int	i, j, n;
	char	src [1024] = "this is a long string of stuff to see what happens";
	char	dest [1024];

	n = strlen (src);	/* how long is src		*/
	i = n - 10;		/* starting point in src	*/
	j = 0;			/* starting point in dest	*/
	/*	check length of src				*/
	if (i < 0)
		i = n;
	for ( ; i < n && src [i] != '\0'; i++) {
		dest [j] = src [i];
		j++;
	}
	dest [j] = '\0';
	printf ("%s\n", src);
	printf ("%s\n", dest);
}

And, when executed

Code:

this is a long string of stuff to see what happens
at happens

Hope this helps some.

piyush.sharma · 08-18-2012, 12:13 AM

It worked for me, try this :

Code:

/*
str : source string
out : output
n : number of characters
*/
void copyString(char *str,char *out,int n)
{
        strcpy(out,(str+strlen(str)-n));
}

KernelJay · 09-27-2012, 07:35 PM

That strcpy should really be strncpy to be safe. strncpy allows you to pass the maximum number of bytes which can be copied so as to protect your destination buffer. Unbounded string manipulation can have disastrous results. For some additional insight into why strcpy is so dangerous, check out my latest blog post on the subject:
VERT Vuln School: Stack Buffer Overflows 101

Part 1: Introducing the Bug
Part 2: Explaining the Stack
Part 3: Exploiting the Bug

Thanks,
Craig