Thread: Problem With Swapping

Hello,
I have written a code to swap two variables of generic types....
I am able to swap numeric values correctly... but when i try to work with character arrays it is swapping but some characters are missed...
Here is the code..

Code:

/***This is generic swap function implementation, which swaps two variables of any data types
    But it uses dynamically allocated buffer rather than stack buffer..   ***/

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

void swap(void *, void *, int);
int main()
{
   int a = 10, b = 20;
   char *s1 = strdup("this is new ........!");
   char *s2 = strdup("im gonna sleep..");
   printf("\n Before swap\n");
   printf(" a = %d\n b = %d\n",a,b);

   swap(&a, &b, sizeof(int));

   printf("\n After swap\n");
   printf(" a = %d\n b = %d\n",a,b);

   printf("\n Swapping strings\n");
   printf("Before Swap\n");
   printf(" s1 = %s\n", s1);
   printf(" s2 = %s\n", s2);

   swap(s1, s2, sizeof(char *));
   printf(" After Swap\n");
   printf(" s1 = %s\n", s1);
   printf(" s2 = %s\n", s2);
}

//accepts pointer to two variables and their size.
void swap(void *vp1, void *vp2, int size)
{
   //temp variable equivalent..a buffer
   char *buffer = (char *)malloc(size);
    //memcpy copies bits from one location to another, memcpy(source, destination, size)
   memcpy(buffer, vp1, size);

   memcpy(vp1, vp2, size);
   memcpy(vp2, buffer, size);

   //now we are done so we need to free dynamically allocated memory
   free(buffer);
}

And also when is declare strings without strdup.. while swapping it gives seg fault... as im moving addresses not manipulating contents of those address then why it is giving seg fault...??

Thnx!!

Originally Posted by MK27

Code:

   swap(s1, s2, sizeof(char *));

You should try printing that out to see exactly what sizeof(char*) is, and consider what the consequences of that are.

How about:

Code:

   int a = strlen(s1), b = strlen(s2);
   swap(s1, s2, (a > b) ? a + 1 : b + 1);

If the inputs are true pointers (to const string literals, or strings created with malloc or strdup), then all you have to do is swap the pointers.

I got it.. i missed &.... it should be swap(&s1,&s2, sizeof(char *)) and it's working fine... as memcpy operates on data pointed to by address.. so for char string i must pass char **...
And that was reason i was getting set fault witout strdup coz i was trying to change contents of constant string which is defined in string table and is read only i guess....

Originally Posted by MK27

NO. YOU ARE WRONG.

Sizeof(char*) is either 4 (32 bit system) or 8 (64 bit system). Of course, if every word you swap is less than that, you are okay. Otherwise this:

Code:

char *buffer = (char *)malloc(size);

is not long enough. However, it might work sometimes anyway if nothing vital gets overwritten by the overrun. In this case such "luck" is unfortunate because you now believe code that appears to work == correctly written safe code, but that is not true.

You are not getting wht i said... i know size of char * is dependent on architecture.... im here swapping addresses pointed by vp1 and vp2 in case of char string... so i need to pass char ** which is done by placing & in front of char arrays... swap(&s1,&s2, sizeof(char *)) so now swap is exchanging addresses stored at s1 and s2...im not swapping actual bits of data... it's just pointers getting swapped... but in case of int it's simply single pointer..... and code works perfectly fine....