Custom strcat() function.

hello;

i am trying to write my own version of strcat() that uses pointers. i dont know why its not working; can someone take a look at this piece of code?

thanks in advance

Code:

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#include <stdio.h>

char *my_strcat(char *a, char *s_a);

int main(void){
    char x1[30] = "a";
    char x2[10] = "boy";
    my_strcat(x1, x2);

    printf("%s", x1 );

}

char *my_strcat(char *a, char *b){

    while (*a++ != '\0');

    while(*b != '\0'){
        *a++ = *b++;
    }
    *a = '\0';
    return a;
}

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You advance a too far.

Code:

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$gcc -g main.c
$ gdb -q a.out
Reading symbols from a.out...done.
(gdb) list
1        #include <stdio.h>
2       
3        char *my_strcat(char *a, char *s_a);
4       
5        int main(void){
6            char x1[30] = "a";
7            char x2[10] = "boy";
8            my_strcat(x1, x2);
9       
10            printf("%s", x1 );
(gdb) b 10
Breakpoint 1 at 0x4005f3: file main.c, line 10.
(gdb) run
Starting program: /home/sc/Documents/a.out

Breakpoint 1, main () at main.c:10
10            printf("%s", x1 );
(gdb) p x1
$1 = "a\000boy", '\000' <repeats 24 times>
(gdb)

---

You append the string OK, but you appended it in the wrong place.

It's a good time to familiarise yourself with a debugger to help you figure out such things for yourself.

Quote:

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Originally Posted by Salem

You advance a too far.

Code:

---

$gcc -g main.c
$ gdb -q a.out
Reading symbols from a.out...done.
(gdb) list
1    #include <stdio.h>
2   
3    char *my_strcat(char *a, char *s_a);
4   
5    int main(void){
6        char x1[30] = "a";
7        char x2[10] = "boy";
8        my_strcat(x1, x2);
9   
10        printf("%s", x1 );
(gdb) b 10
Breakpoint 1 at 0x4005f3: file main.c, line 10.
(gdb) run
Starting program: /home/sc/Documents/a.out

Breakpoint 1, main () at main.c:10
10        printf("%s", x1 );
(gdb) p x1
$1 = "a\000boy", '\000' <repeats 24 times>
(gdb)

---

You append the string OK, but you appended it in the wrong place.

It's a good time to familiarise yourself with a debugger to help you figure out such things for yourself.

---

Thanks for your reply. i have a question though. which debugger do you think is appropriate for someone who has zero
experience in debugging. i am a using a laptop running windows

Quote:

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Originally Posted by thagulf2017

Thanks for your reply. i have a question though. which debugger do you think is appropriate for someone who has zero
experience in debugging. i am a using a laptop running windows

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If your compiler comes with a debugger, then that is the one you should use. If not, then maybe install the gcc compiler and debugger.

Note that the return value of strcat is a pointer to the (first character of the) resulting string, i.e., it returns a pointer equal to the first parameter's value. Since you modify the parameter a before returning it, my_strcat therefore differs from strcat in this respect.

thank you all for replies, it works now :)

Minor thoughts and nitpicks.

> char *my_strcat(char *a, char *b)

Granted this is a custom strcat, but the signature doesn't match strcat. In particular, the second argument does not need to be modifiable and best practice would be to make it const to avoid inadvertent changes.

> while (*a++ != '\0');

I'm not a huge fan of directly modifying a parameter, especially when the interface of the function returns the parameter unchanged. Also, while I'll contradict myself immediately, one-liner statements with a lot happening are more difficult to wrap your head around as well as debug.

> *a = '\0';

This is unnecessary if you copy it in your loop.

Here's an example of a standard-conforming strcat for comparison purposes:

Code:

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char *strcat(char * restrict dst, const char * restrict src)
{
    char *p = dst;

    /* Find the terminating null character in dst */
    while (*p)
    {
        ++p;
    }

    /* Copy from the null to append src */
    while (*p++ = *src++)
        ;

    return dst;
}

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