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Top Forums Programming Reg: strncpy function
# 8  
Old 10-10-2006
I stayed out of this, but memcpy and strncpy don't append a trailing '\0', that' why you should consider memset()

Run this code and see if you see a trailing '\0' after "HI THERE".
Code:
#include <strings.h>
int main()
{
        char tmp[32];
        memset(tmp,'%',32);
        strncpy(tmp,"HI THERE",3);
        {
                char *p=tmp;
                int i=0;
                for(i=0;i<32;i++,p++) printf("%2d ",*p);
                printf("\n");
        }
        return 0;
}


This is what I get:
Code:
kcsdev:/home/jmcnama> cat t.c
#include <strings.h>
int main()
{
        char tmp[32];
        memset(tmp,'%',32);
        strncpy(tmp,"HI THERE",3);
        {
                char *p=tmp;
                int i=0;
                for(i=0;i<32;i++,p++) printf("%2d ",*p);
                printf("\n");
        }
        return 0;
}
kcsdev:/home/jmcnama> cc t.c
kcsdev:/home/jmcnama> a.out
72 73 32 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37
kcsdev:/home/jmcnama>

# 9  
Old 10-10-2006
Quote:
Originally Posted by Corona688
And this has what to do with my example?

Like I said, there is always the potential for garbage whenever you overwrite a string of unknown contents. Should you fill all string buffers with nulls, every time you overwrite them? That could be an awful lot of nulls.
ok.. lets take you dont fill them using memset..

once you prepare the string .. with strncpy, strcpy (some func)

how do u notify the string termination ?
# 10  
Old 10-10-2006
memset and strncpy are the only standard C string "copy" functions that don't null-terminate the destination string.

And Corona's example of
Code:
str[0]='\0';

doesn't handle strncpy, it allows buffer overrun, because there is no trailing '\0'
except by luck of what happened to be in the memory allocated on the stack for str1...
# 11  
Old 10-10-2006
Code:
strncpy(dst,src,n-1);
dst[n]='\0';

Covers all situations. If strncpy has enough space, it will leave a null terminator at or before the index n, and putting the terminator at the end will do nothing. If it does not have enough space, then by definition it will need one at index n.
Last edited by Corona688; 10-10-2006 at 03:15 PM..
# 12  
Old 10-10-2006
Quote:
strcpy(), strncpy(), strlcpy()
The strcpy() function copies string s2 to s1, including the
terminating null character, stopping after the null charac-
ter has been copied. The strncpy() function copies exactly n
bytes, truncating s2 or adding null characters to s1 if
necessary. The result will not be null-terminated if the
length of s2 is n or more. Each function returns s1.

The strlcpy() function copies at most dstsize-1 characters
(dstsize being the size of the string buffer dst) from src
to dst, truncating src if necessary. The result is always
null-terminated. The function returns strlen(src). Buffer
overflow can be checked as follows:

if (strlcpy(dst, src, dstsize) >= dstsize)
return -1;

This is my understanding of strncpy. As long as it copies n + m characters into the destination from a source that is n bytes long, you're okay.
n-1 and it does not null-terminate.
# 13  
Old 10-10-2006
Quote:
The strncpy() function copies exactly n
bytes, truncating s2 or adding null characters to s1 if
necessary. The result will not be null-terminated if the
length of s2 is n or more.
Quote:
This is my understanding of strncpy. As long as it copies n + m characters into the destination from a source that is n bytes long, you're okay.
n-1 and it does not null-terminate.
i feel the statements are contradictory !!! Smilie Smilie Smilie
# 14  
Old 10-10-2006
That's exactly my point, Jim. If you're copying a string into a buffer that's too small, it won't be null terminated. If you're copying a string that's smaller than the buffer, it will be null-terminated.

So, just put a null on the end of the buffer, and it will always be null terminated no matter what the size of the input string.

As usual, there's no need to nullify the whole buffer. For small buffers you can get away with it, but it's not good to get into the habit of it.
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