Thread: Whats wrong with the Output?

Hi, I created a function which performs similarly to strcat().

Code:

#include<stdio.h>
#include<string.h>
char xstrcat(char *s,char *q,int x);
int main()
{
  char a[]="Shabbir",b[]="Hussain";
  char *c[50];
  int x;
  
  x=strlen(a);
  xstrcat(a,b,x);
  printf("\n\n&#37;s\n\n",a);
  return 0;
}

char xstrcat(char *s,char *q,int x)
{
  int i;
  for(i=0;i<=x;i++){
    if(*s=='\0'){
      *s=' ';
      s++;
      while(*q!='\0'){
	*s=*q;
	++s;
	++q;
      }
    }
    else{
      ++s;
    }
  }
  *s='\0';
  return 0;
}

I got the desired Output, but also something else.

Code:

shabbirhussain@shabbirhussain:~/c$ ./c


Shabbir Hussain

*** stack smashing detected ***: ./c terminated
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x48)[0xb7ead138]
/lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x0)[0xb7ead0f0]
./c[0x8048495]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe0)[0xb7dd6450]
./c[0x80483a1]
======= Memory map: ========
08048000-08049000 r-xp 00000000 08:06 49660      /home/shabbirhussain/c/c
08049000-0804a000 rw-p 00000000 08:06 49660      /home/shabbirhussain/c/c
0804a000-0806b000 rw-p 0804a000 00:00 0          [heap]
b7dbf000-b7dc0000 rw-p b7dbf000 00:00 0 
b7dc0000-b7f09000 r-xp 00000000 08:06 286938     /lib/tls/i686/cmov/libc-2.7.so
b7f09000-b7f0a000 r--p 00149000 08:06 286938     /lib/tls/i686/cmov/libc-2.7.so
b7f0a000-b7f0c000 rw-p 0014a000 08:06 286938     /lib/tls/i686/cmov/libc-2.7.so
b7f0c000-b7f0f000 rw-p b7f0c000 00:00 0 
b7f16000-b7f20000 r-xp 00000000 08:06 269344     /lib/libgcc_s.so.1
b7f20000-b7f21000 rw-p 0000a000 08:06 269344     /lib/libgcc_s.so.1
b7f21000-b7f24000 rw-p b7f21000 00:00 0 
b7f24000-b7f25000 r-xp b7f24000 00:00 0          [vdso]
b7f25000-b7f3f000 r-xp 00000000 08:06 269299     /lib/ld-2.7.so
b7f3f000-b7f41000 rw-p 00019000 08:06 269299     /lib/ld-2.7.so
bf8a6000-bf8bb000 rw-p bffeb000 00:00 0          [stack]
Aborted

Is there something wrong with the compiler? What does this whole thing mean?

Yes, you are packing both your names + a space in the array that has only just got enough room for the one name in variable a.

Code:

char a[]="Shabbir"

will have exactly the right space to hold 'S', 'h', 'a', 'b', 'b', 'i', 'r', '\0' - which means that when concatenate your strings, you go over into space that doesn't belong to the string.

--
Mats

Originally Posted by shabbirhussain

And how do I avoid this problem?

Make the destination string long enough, e.g.

Code:

char a[100]="Shabbir"

--
Mats