Thread: My strcat

Hi All,
I am a beginner of C Programming. I wrote a strcat function of my own. Please let me know your comments:

Code:

#include <stdio.h>


void mystrcat(char *, char *, char *);


int main()
{
    char s1[] = "Hello";
    char s2[] = "World";
    char catstring[sizeof(s1) + sizeof(s2)];
       
    mystrcat(s1, s2, catstring);
    printf("%s", catstring);
    getch();
}


void mystrcat(char *s1, char *s2, char *catstring)
{
    
    int i=0, k=0;
    
    while(*s1 != '\0')
    {
           catstring[k] = *s1;
           k++;
           s1++;
           
    }      
    
    while(*s2 != '\0')
    {
           catstring[k] = *s2;
           k++;
           s2++;
    }
    catstring[k] = '\0';
       
}

Originally Posted by ledow

What if catstring is passed as NULL?

i is unused.

Modified the code ... Please let me know if this seems ok ...

Code:

#include <stdio.h>


void mystrcat(char *, char *, char *);


int main()
{
    char s1[] = "Hello";
    char s2[] = "";
    char catstring[sizeof(s1) + sizeof(s2)];
       
    mystrcat(s1, s2, catstring);
    printf("%s", catstring);
    if (*catstring == 0)
    {
        printf("String is Empty\n");
    }
    getch();
}


void mystrcat(char *s1, char *s2, char *catstring)
{
    
    int k=0;
    
    while(*s1 != '\0')
    {
           catstring[k] = *s1;
           k++;
           s1++;
           
    }      
    
    while(*s2 != '\0')
    {
           catstring[k] = *s2;
           k++;
           s2++;
    }
    catstring[k] = '\0';
       
}

It still won't fix the problem if one of the strings are NULL, as you're still dereferencing the pointers without checking their validity.

Also, your loops could be shortened significantly with no functional change:

Code:

while (*s1)
    catstring[k++] = *s1++;
while (*s2)
    catstring[k++] = *s2++;

Originally Posted by memcpy

It still won't fix the problem if one of the strings are NULL, as you're still dereferencing the pointers without checking their validity.

Also, your loops could be shortened significantly with no functional change:

Code:

while (*s1)
    catstring[k++] = *s1++;
while (*s2)
    catstring[k++] = *s2++;

I changed the strings as follows:

Code:

char s1[] = "Hello";
char s2[5];

It seems to work fine. Also note that I am using following line in the code

Code:

 catstring[k] = '\0';

Should it not suffice to handle Null strings?

> I changed the strings as follows:
That's not going to help. 's2' isn't NULL, it's five bytes of stack memory that have no meaningful value assigned to them. Checking for NULL means that your function will not accept NULL as an argument:

Code:

mystrcat(s1, s2, NULL);

> Should it not suffice to handle Null strings?
No, all that does is null terminates the string. You need this, but it doesn't check for NULL.

Hmmm ... I understand ...