Strcat with pointers and Segmentation Fault

I'm trying to code a strcat with pointers:

Code:

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#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;
     
}

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debug

Code:

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  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++) )

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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
*/

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Quote:

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Not surprising. You need adequate space for the result.

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

I'm getting only Thames and not ThamesThames as an output. I know it has something to do with

Code:

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  while(*s++ != '\0')
  ;

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because of that debug I posted before. However, I thought the expression was correct.
[/code]

Code:

---


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

#ifndef MAXSIZE
  #define MAXSIZE 1000
#endif 


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


#ifndef MAXSIZE
  #define MAXSIZE 1000
#endif 


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

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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;
     
}

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I observed this:

Code:

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while(*s != '\0')
    *s++;

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It is unnecessary to dereference s if your intention is only to increment it, i.e.,

Code:

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while (*s != '\0')
    s++;

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