c program using pointers of reverse

Question

Why it's not functioning? compiler says its returning local address. any help how to make it it correct. i saw other reverse string programs but i didn't find any problem in it.

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

char *reverse(char *str);

void main()
{
    char str[]="jamesbond";
    int len=strlen(str);
    char *ptr;
    ptr=reverse(str);
    printf("the reversed string is :%s",ptr);
}

char *reverse(char *str)
{ 
    int i,j;
    int len=strlen(str);
    char qtr[len];

    for( i=len-1;i<=0;i--)
    { 
        for( j=0;j<len-1;j++)
            qtr[j]=*(str+i);
    }
    return qtr;
}

You cant return `qtr`, it is not valid after `reverse` exits. — imreal, Jan 24 '13 at 17:58
Please do a minimum of research before asking a question. Searching for the error message in Google will yield a ton of helpful results. — netcoder, Jan 24 '13 at 18:00
Also, check [this](http://faq.cprogramming.com/cgi-bin/smartfaq.cgi?id=1043284376&answer=1044841143) please. — Grieverheart, Jan 24 '13 at 18:06
http://stackoverflow.com/questions/198199/how-do-you-reverse-a-string-in-place-in-c-or-c — qulinxao, Jan 24 '13 at 18:12
In short, you are returning garbage. Since you have pointers in C why not make reverse prototype like int reverse(char *instr, char *reversed) — Andro, Jan 24 '13 at 18:17
Note that this question is merely a repost of Kundan's original post here: http://stackoverflow.com/questions/14504993/program-to-reverse-a-string — phonetagger, Jan 24 '13 at 18:45

score 0 · Answer 1 · answered Jan 24 '13 at 17:59

0

returning a stack variable(or array) (here qtr) from a function is undefined behavior. It will be overwritten.

Solution is to malloc() the array and return that instead, then free() it when you're done using it.

answered Jan 24 '13 at 17:59

Aniket Inge

25,375
5
50
78

Carl Norum · Accepted Answer · 2013-01-24T18:31:40.317

0

qtr is a local array, and you're returning a pointer to it. You can't do that. You need to allocate dynamic memory and return that:

char *qtr = malloc(len + 1);

Note that you will need to free() this memory sometime. Alternately, just do the reversal in place:

char *start = str;
char *end = str + strlen(str) - 1;
char tmp;

while (start < end)
{
   tmp = *start;
   *start++ = *end;
   *end-- = tmp;
}

edited Jan 24 '13 at 18:31

answered Jan 24 '13 at 17:59

Carl Norum

219,201
40
422
469

Carl Norum: you want to say that we can return a local address only if its dynamically allocated by using malloc(). – Kundan Negi Jan 24 '13 at 18:04
If you allocate it using `malloc` it's not a local address. – Carl Norum Jan 24 '13 at 18:05
Carl Norum: i did as you said of using malloc.but compiler gives following error warning: incompatible implicit declaration of built-in function ‘malloc’ [enabled by default] – Kundan Negi Jan 24 '13 at 18:31
You need to `#include ` to use `malloc()`. I think you would benefit from reading some beginner tutorials to get up to speed on the basics. – Carl Norum Jan 24 '13 at 18:32
Carl Norum: ya you are right. i am a beginner. i just enrolled for C a week ago. Can u recommend a nice book for me – Kundan Negi Jan 24 '13 at 18:33
Carl Norum: Now there are no errors but there is no answer. only the printed statement is: the reverse string is me@home$ – Kundan Negi Jan 24 '13 at 18:42
Well, your code has other bugs, too. Try using a debugger to step through and see what's up. Hint - your nested loops aren't right. – Carl Norum Jan 24 '13 at 18:43
what debugger can i use for ubuntu linux – Kundan Negi Jan 24 '13 at 18:53
`gdb` is pretty much standard. – Carl Norum Jan 24 '13 at 18:54

score 0 · Answer 3 · answered Jan 24 '13 at 17:59

0

qtr is being allocated on the stack. Memory which goes out of scope as soon as your function returns. You could declare qtr as static as a workaround.

answered Jan 24 '13 at 17:59

itsme86

19,266
4
41
57

score 0 · Answer 4 · answered Jan 24 '13 at 18:04

The condition for the first for loop must be change to

i >= 0



for( i=len-1;i>=0;i--)
  { for( j=0;j<len-1;j++)
    qtr[j]=*(str+i);

   }

And since you are not passing the pointer to the resulting array.

It is better to malloc for qtr

qtr = malloc(sizeof(strlen(str));

Also

 int len=strlen(str);

in your main() is an unwanted computation.

score 0 · Answer 5 · answered Jan 24 '13 at 18:33

I fount this pice of code somewhere. Looks legit. And if you don't want to ruinn you input string just deep copy the pointer (memcpy()) and insert it in.

char* strrev( char* s)
  {
  char  c;
  char* s0 = s - 1;
  char* s1 = s;

  /* Find the end of the string */
  while (*s1) ++s1;

  /* Reverse it */
  while (s1-- > ++s0)
    {
    c   = *s0;
    *s0 = *s1;
    *s1 =  c;
    }

  return s;
  }

c program using pointers of reverse

5 Answers5