strncpy and strcat garbage characters in C

Question

I tried to write a programme which copies the first k char from a string s1 and then concatenates the result with the rest of string s2 starting from a position i then concatenates the rest of s1 in the result. Yet I had some problems with strncpy. It shows some weird characters in console like @.

Here's my code:

char* strninsert(char *s1, char *s2, int k,int i)
{
    int n=strlen(s1)+strlen(s2)+10; // if i put for exemple 1000 it works
    char *result=(char*)malloc(n*sizeof(char));
    result[0]='\0';
    strncpy(result,s1,k);
    strcat(result,(s2+i));
    strcat(result,(s1+k));
    puts(result);
    return(result);
}

int main()
{
    int lens;
    printf("size string 1 ");
    scanf("%d",&lens);
    char *s=(char*)malloc((lens+1)*sizeof(char));
    getchar();
    fgets(s,lens+1,stdin);

    int lenx;
    printf("enter size string 2 ");
    getchar();
    scanf("%d",&lenx);
    char *x=(char*)malloc((lenx+1)*sizeof(char));
    getchar();
    fgets(x,lenx+1,stdin);

    int lentemp=lenx+lens;
    char *temp=(char*)malloc((lentemp+1)*sizeof(char));

    temp=strninsert(s,x,2,3);
    puts(temp);
    return(0);
}

It shows me weird characters after the strncpy instruction like for example poes@<line.

[strncpy man page says](http://linux.die.net/man/3/strncpy): "Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated". That is, if the value `k` in your code is less than `strlen(s1)+1` then the resulting contents in `result` will not be NUL terminated. — kaylum, Nov 29 '15 at 22:51
There are many problems with this code, such as memory leaks, casting the result of `malloc` (it's a disease), and bad formatting. What is the purpose of the `k` and `i` variables? Because they're the main cause of this issue. — user4520, Nov 29 '15 at 22:59
@kaylum thank you so much so i have to add result[k]='\0' just after the instruction strncpy ? it should work fine — programmer, Nov 30 '15 at 22:49
@szczurcio i'm open to any solutions or suggestions, thank you — programmer, Nov 30 '15 at 22:51
`snprintf(result, n, "%.*s%s%s", k, s1, s2+i, s1+k);` . It'd also be a good idea to validate that `i` and `k` are not past the end of `s2` and `s1` — M.M, Nov 30 '15 at 23:01

kaylum · Accepted Answer · 2015-11-29T23:02:15.800

strncpy man page says:

Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated.

That is, if the value k in your code is less than strlen(s1)+1 then the resulting contents in result will not be NUL terminated. One possible fix is to change your code thus:

/* Sanity check. */
if (k > strlen(s1)) {
    k = strlen(s1);
}

strncpy(result , s1, k);
result[k] = '\0';

Also, not directly related to your problem but you have memory leaks in your code.

In main the temp variable received a malloc buffer which is then immediately lost as temp is overwritten by the strninsert result.
The result of strninsert is dynamic memory which is not freed anywhere.

Thank you so much for your time, this was very useful to me everything worked fine. thank's alot — programmer, Nov 30 '15 at 23:03

score 0 · Answer 2 · answered Nov 30 '15 at 02:17

As previously answered by kaylum, the result of strncpy is not necessarily a string. This poses a hazard when passing it to strcat, as strcat requires that both arguments definitively be a string.

The nicest solution to your problem I can think of involves using sprintf, like so:

void solution(char *destination, char *x, int x_size, char *y) {
    sprintf(destination, "%*.s%s", x_size, x, y);
}

strncpy and strcat garbage characters in C

2 Answers2