Thread: Strcat with pointers and Segmentation Fault

I'm trying to code a strcat with pointers:

Code:

 

#include <stdio.h> 

void Strcat(char* s, char* t) 
{ 
   while(*s++ != '\0')
   ; 
   while( (*s++ = *t++) ) 
   ;      
}     

int main(void) 
{  
   char* s1 = "Thames"; 
   char* s2 = "Thames";     
  
   Strcat(s1, s2);
   
   printf("The concatenated string is %s\n", s1); 
   return 0;
      
}

debug

Code:

  
   gdb -q strcat2
Reading symbols from /home/ethereal/C/Strings/strcat2...done.
(gdb) break 5
Breakpoint 1 at 0x400500: file strcat2.c, line 5.
(gdb) break 7
Breakpoint 2 at 0x400516: file strcat2.c, line 7.
(gdb) run
Starting program: /home/ethereal/C/Strings/strcat2 

Breakpoint 1, Strcat (s=0x400680 "Thames", t=0x400680 "Thames") at strcat2.c:5
5       while(*s++ != '\0')
(gdb) c
Continuing.

Breakpoint 2, Strcat (s=0x400687 "", t=0x400680 "Thames") at strcat2.c:7
7       while( (*s++ = *t++) ) 
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400522 in Strcat (s=0x400687 "", t=0x400680 "Thames") at strcat2.c:7
7       while( (*s++ = *t++) )

Not surprising. You need adequate space for the result.

Code:

#include <stdio.h>
#include <string.h>

int main()
{
   char s1[100] = "Thames";
   const char *s2 = "Thames";
   

   strcat(s1, s2);

   puts(s1);

   return 0;
}

/* my output:
ThamesThames
*/

Not surprising. You need adequate space for the result.

I think I've been trying to do magic with that code : \

Oh, well, after *s++ != '\0' is actually true, the postfix increment has moved the pointer one past the zero terminator. In effect it isn't over written in the next loop.

I see. I fixed the problem. Many thanks.

Code:

 
#include <stdio.h> 

#ifndef MAXSIZE 
  #define MAXSIZE 1000
#endif  


void Strcat(char* s, const char* t) 
{ 
   while(*s != '\0')
    *s++;
   
   while( (*s++ = *t++) ) 
   ;      
}     

int main(void) 
{  
   char  s1[MAXSIZE] = "Thames"; 
   const char* s2 = "Thames";     
  
   Strcat(s1, s2);
   
   printf("The concatenated string is %s\n", s1); 
   return 0;
      
}