Reversing a C string function crashes?

Question

I'm trying to write a C function to reverse a passed in C style string (ie char *) and return the char pointer of the reversed string. But when I run this in VS2012, nothing is printed in terminal and "main.exe has stopped working" msg shows up.

#include <stdio.h>
#include <string.h>
char * rrev_str(char * str )
{
    char *revd_str=""; //I tried char revd_str []="" error: stack around "revd_str" is corrupted
    int i,r;
    int str_len=strlen(str);
    for (i = str_len-1, r=0; i >=0; i--,r++)
    {
        revd_str[r]= str[i];
    }
    return revd_str;
}

int main(int argc, char* argv[])
{
   char str1 [] ="STEETS";
   char str2 [] ="smile everyday!";

   //reverse "chars" in a C string and return it
   char * rev_string=rrev_str(str1);
}

stackoverflow.com/questions/8784099/… Same problem as this one .. — JRK
– JRK, Commented May 17, 2013 at 18:01
Run-Time Check Failure #2 - Stack around the variable 'revd_str' was corrupted. — jerryh91
– jerryh91, Commented May 17, 2013 at 18:24

mshildt · Accepted Answer · 2013-05-17 18:13:08Z

2

The problem here is three fold. First you aren't allocating enough space for the reversed string, and secondly you are returning a pointer to a local variable in rrev_str(), and thirdly you're modifying a string literal. You need to allocate space for revd_str on the heap:

char * rrev_str(char * str )
{   
    int i,r;
    int str_len=strlen(str);

    char *revd_str=malloc(str_len + 1); 
    memset(revd_str, 0, str_len + 1);

    for (i = str_len-1, r=0; i >=0; i--,r++)
    {
        revd_str[r]= str[i];
    }
    return revd_str;
}

edited May 17, 2013 at 18:13

answered May 17, 2013 at 18:01

mshildt

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3 Comments

Femaref Over a year ago

You'll need to assign the terminator as well, as malloc doesn't guarentee a zero'ed memory location.

jerryh91 Over a year ago

Thanks that worked. With 1 small syntax edit: memset's 3rd arg expects a variable and no other operation done with that var when passed thru. so instead of str_len+1, it expects a single variable storing this val. So I put str_len+1 in a temp_var and passed it to memset

Navnath Godse Over a year ago

Yes it will work. But you need to free heap memory when it is not require. It may be memory leak if you not free it in future.

Navnath Godse · Accepted Answer · 2013-05-17 18:50:25Z

Problem:

You are accessing invalid memory address.
revd_str is pointing to literal constant string of length 1 and you are accessing it beyond the length which is invalid.

Solution:

Create char array of require length (statically or dynamically).
Reverse the given string.
Pass 2nd param as destination string
syntax: char * rrev_str(char * src, char *dest);

Reverse the given string

char * rrev_str(char * str )
{
   int start = 0;
   int end = strlen(str) - 1;
   char temp;

    for (; start < end; start++ ,end--)
    {
        temp = str[start];
        str[start] = str[end];
        str[end] = temp;
    }
    return str;
}

int main(int argc, char* argv[])
{
   char string [] ="smile";

   //reverse "chars" in a C string and return it
   char * rev_string = rrev_str(string);

   printf("%s",rev_string);
}

Pass 2nd param as destination string

char * rrev_str(char * src, char *dest)
{
   int srcLength = strlen(src);
   int destLength = strlen(dest);
   int i;
   // Invalid destination string
   if (srcLength > destLength)
   {
        return NULL;
   }

   dest[srcLength] = '\0';
   srcLength--;
    for (i=0; srcLength >= 0;i++, srcLength--)
    {
        dest[i] = src[srcLength];
    }

 return dest;
}

int main(int argc, char* argv[])
{
   char string [] ="smile";
   char revString[20];  

   //reverse "chars" in a C string and return it
   char * rev_string = rrev_str(string, revString);

    printf("%s",rev_string);
}

Community · Accepted Answer · 2020-06-20 09:12:55Z

0

What! you are doing..

char *revd_str=""; // Creating String Literal which can't be modified because they are read only  
char *revd_str[]=""; // Creating Char Array of Size Zero.

So Solution are

Either take reference of your string

char *revd_str = strdup(str);

Or create dynamic char array

char *revd_str = (char*) malloc (strlen(str)+1);

your program will run fine. logic is incorrect for reversing so modify it. A sample solution is given below

char * rrev_str(char * str )
{
    char *revd_str=strdup(str);
    int i;  // no need for extra 'int r'
    int str_len=strlen(str);
    for (i = 0; i < str_len/2; i++)
    {
        char temp = revd_str[i];
        revd_str[i]= revd_str[str_len - 1 -i];
        revd_str[str_len - 1 -i] = temp;
    }
    return revd_str;
}

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jan 13, 2014 at 6:03

Jai

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2 Comments

user1055604 Over a year ago

Or perhaps use strdup to get a duplicate of the same size. Also, char *revd_str=str does not create a copy of str, it overwrites str.

Jai Over a year ago

thank you for pointing it out. I was taken wrong it as change in given string. I have corrected it.

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