Segmentation fault (core dumped) in a C program

Question

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

char *Change(char *str,int start,int end)
  {
  if(start==end || start>end){
    return str;
  }

  else{
    char temp=str[start];
    str[start]=str[end];
    str[end]=temp;
    return(Change(str,start++,end--));
  }
  return str;
}


char *Reverse(char *str)
{
   int length=strlen(str);
  return(Change(str,0,length-1));
}

int main()
{
   printf("%s\n",Reverse("program"));
   return 0;
}

I am trying to write a recursive function to reverse a string,but it comes the linking error.Please help.I had tried so many times and searched in the Internet,but it can't help. I guess the most probably place causing the problem is in function Change,but I can't solve with it.

Have you run it in a debugger at all?... You can catch a seg fault and look at where your code has gone wrong. — Joe, Nov 20 '13 at 08:39
Where is `temp` declared in `Reverse()` ? Please post the actual code. — zakinster, Nov 20 '13 at 08:40
See also http://stackoverflow.com/questions/164194/why-do-i-get-a-segmentation-fault-when-writing-to-a-string or http://stackoverflow.com/questions/1614723/why-is-this-string-reversal-c-code-causing-a-segmentation-fault — nos, Nov 20 '13 at 08:40
@luojiebin You actually have two errors in your code (see my answer), if you correct both, your code will work. — zakinster, Nov 20 '13 at 09:14

zakinster · Accepted Answer · 2013-11-20T09:12:11.893

You're passing a string literal (which is a const char*) to Reverse(), since you can't modify a string literal, your program generates a segmentation fault.

You'll either need to pass a modifiable string to Reverse():

char myStr[] = "program";
printf("%s\n",Reverse(myStr));

Or you can make a copy of the input string in Reverse():

char *Reverse(const char *str)
{
   int length=strlen(str);
   char* temp = malloc(length);
   strcpy(temp, str);
   return(Change(temp,0,length-1));
}

In that case, you'll need to free the string returned by Reverse():

char* reverseStr = Reverse("program");
printf("%s\n", reverseStr );
free(reverseStr);

Also in change(), start++ end end-- will return the value before they are incremented, you need to use either ++start or simply start+1 since you won't use those variables anymore:

return(Change(str,start+1,end-1));

haccks · Answer 2 · 2013-11-20T08:49:03.733

0

Simple explanation: You can't modify a string literal. One more thing is that the statement

return(Change(temp,0,length-1));

will cause compilation error because neither temp is defined in the function Reverse nor it is globally defined.

edited Nov 20 '13 at 08:49

answered Nov 20 '13 at 08:42

haccks

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Segmentation fault (core dumped) in a C program

2 Answers2