C code to reverse a string - including NULL character at the end of string

Question

1.) Is it possible to reverse a string including the NULL character (which means that “abcd” is represented as five characters, including the null character.)

2.) In my current implementation, that doesn't take 1.) into account, I am getting segmentation error during swapping. ie while assigning: *str = *end;

void reverse(char *str)
    {
        char * end = str;
        char tmp;
        if (str)
        {  // to handle null string
            while (*end)
            {  // find the end character
                ++end;
            }
            --end; // last meaningful element
            while (str < end) // terminal condition: str and end meets in the middle
            {   tmp = *str; // normal swap subroutine
                *str = *end; // str advance one step
                *end = tmp;   // end back one step

                str++;
                end-- ;
            }
        }
        return;
    }

There is no `NULL` character. `NULL` is the _null pointer_ constant. You might mean the ASCII-`NUL` terminator character `'\0'`. — too honest for this site, Jul 21 '15 at 18:52
How do you call this function when you get a segmentation error? — wimh, Jul 21 '15 at 18:58
@arshdeep kaur It seems that you are trying to reverse a string literal. You may not reverse string literals. Instead declare a character array with the original string. — Vlad from Moscow, Jul 21 '15 at 19:03
If you reverse including the `NUL` terminator, you will effectively getting an empty string. What worth would this be? — too honest for this site, Jul 21 '15 at 19:05
You are getting a segmentation fault probably because you are trying to reverse constant string. Code like this: char *str = "abcd"; str[0] = 'x'; will crash, because "abcd" is not writable. — nsilent22, Jul 21 '15 at 19:08
possible duplicate of [Why do I get a segmentation fault when writing to a string initialized with "char \*s" but not "char s\[\]"?](http://stackoverflow.com/questions/164194/why-do-i-get-a-segmentation-fault-when-writing-to-a-string-initialized-with-cha) — Dmitri, Jul 21 '15 at 19:10
Your function should take two arguments: `const char orig[], char rev[]`. Better: three: `const char orig[], size_t rev_len, char rev[rev_len]` — too honest for this site, Jul 21 '15 at 19:10
@Olaf I suggest ten arguments. The more arguments the better!:) — Vlad from Moscow, Jul 21 '15 at 19:18
@Olaf no - if you reverse a NUL terminator, you get an 0xFF terminator;) — Martin James, Jul 21 '15 at 19:24
@MartinJames: Depends if `char` is signed or unsigned and you use 2s complement :-P — too honest for this site, Jul 21 '15 at 19:28
For future reference: With "I am getting segmentation error during swapping", also post the **calling code and input** that went into the error. — chux - Reinstate Monica, Jul 21 '15 at 19:46

Vlad from Moscow · Answer 1 · 2015-07-21T19:36:10.853

2

Your function is correct. It seems that the problem is that you are trying to reverse a string literal. You may not change string literals. They are immutable. Any attemp to change a string literal results in undefined behaviour of the program.

From the C Standard (6.4.5 String literals)

7 It is unspecified whether these arrays are distinct provided their elements have the appropriate values. If the program attempts to modify such an array, the behavior is undefined

Only take into account that it would be better to write

if ( *str )

instead of

 if (str)

Or if you want to check that the poinetr is not NULL then

if ( str && *str )

In this case this decrement

--end;

will be valid even if the original string is empty.

Nevertheless the function itself could be defined the following way as it is shown in the demonstrative program

#include <stdio.h>

char * reverse( char *s )
{
    char *last = s;

    while ( *last ) ++last;

    if ( last != s )
    {        
        for ( char *first = s; first < --last; ++first )
        {
            char c = *first;
            *first = *last;
            *last = c;
        }
    }

    return s;
}    

int main( void )
{
    char s[] = "Hello arshdeep kaur";

    puts( s );
    puts( reverse( s ) );
}

The program output is

Hello arshdeep kaur
ruak peedhsra olleH

edited Jul 21 '15 at 19:36

answered Jul 21 '15 at 19:10

Vlad from Moscow

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1

Upvoted, this is the only answer to mention that modifying string literals is UB. – user4520 Jul 21 '15 at 19:21
@szczurcio Thanks. You are a kind person. – Vlad from Moscow Jul 21 '15 at 19:22
Note: OP's code does `--end;` which is UB should `str = ""`. So not quite "Your function is correct". – chux - Reinstate Monica Jul 21 '15 at 19:30
Nice avoidance of `int` length and index, which should be `size_t` if used - up-vote – chux - Reinstate Monica Jul 21 '15 at 19:36
@chux Thanks. I though that there is if ( *str ) :) – Vlad from Moscow Jul 21 '15 at 19:37
Works perfect. Thanks. There was another problem with my code - The pointer of the reversed string was pointing at the last character by the end of it. This problem has been taken care of in your code. Thanks again. – arshdeep kaur Jul 22 '15 at 10:42

Isaiah · Answer 2 · 2015-07-21T20:24:25.190

I'm quite certain you can. You just need the length of the string and be aware to test NUL.
Strings can and probably should be thought of as character arrays. In particular, trying to direction assign a string literal to an already initialized string is an invalid operation.

An Example:

Here is one way to reverse a string:

void reverse(char *str) {
  // First calculate the length
  unsigned int length = 0;
  int i = 0;
  for (; str[i] != '\0'; i++) {
    ++length;
  }
  ++length;
  // Allocate temporary storage
  char tmp = malloc(length);
  int x = 0;
  // Loop through starting at the end and go backwards
  // It is perfectly legal to change the characters themselves, just not the pointer
  for (i = length - 1; i >= 0; i++, x++) {
    if (str[i] != '\0') {
      tmp[x] = str[i];
    }
  }
  tmp[length - 1] = '\0';
  // Reassign to the given argument
  for (i = 0; i < length; i++) {
    str[i] = tmp[i];
  }
  // Free memory
  free(tmp);
}

Like the other answer stated, you're trying to do something that is left as undefined behavior in the standard.

score 0 · Answer 3 · answered Jul 21 '15 at 19:10

Calling your code like this:

int main()
{
    char a[]="abcd";
    int i;
    reverse(a);
    for (i=0;i<5;i++) {
        printf("a[%d]=%02x\n",i,a[i]);
    }
}

Outputs this:

a[0]=64
a[1]=63
a[2]=62
a[3]=61
a[4]=00

So you're probably passing in a string literal (i.e. char *a="abcd";). These literals are commonly stored in a read-only section of memory, which is likely why it is core dumping.

That being said, doing a reversal including the null character is not of much practical usage when you're dealing with strings.

score 0 · Answer 4 · edited May 08 '20 at 13:45

0

You can try this :

void reverse(char *str) {
    char *end = str;
    char tmp;
    if (str) {
        while (*end){
            ++end;
        }
        --end;
        while (str < end) {
            tmp = *str;
            *str++ = *end;
            *end-- = tmp;
        }
    }
}

edited May 08 '20 at 13:45

Piotr Labunski

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answered May 08 '20 at 11:34

luminien

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C code to reverse a string - including NULL character at the end of string

4 Answers4

An Example: