Void Pointers do not give the expected result

Question

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Hi,
I am trying to assign a string to a pointer of type void but the result is fatal.
The below is the code is compiled and ran on VC++5.0 for your reference. Please help me fix this.

C#

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

int main()
{
    char s1[] = "Hi Thomas";
    void* vpTmp;
    char* s2;
    int idx = 0;


    vpTmp = ( char * ) malloc( strlen( s1 ) + 1 );
    while( s1[idx] != '\0' )
    {
        *( char * )vpTmp = s1[idx++];
        ( (int *)vpTmp )++;
    }
    ( (int * )vpTmp )++;
    *( char * )vpTmp = '\0';
    idx++;
    (int * )vpTmp = ( int * )vpTmp - idx;
    printf("The string is : %s\n", vpTmp );
    s2 = ( char * ) vpTmp;
    printf("The string is: %s\n", s2 );
    return 0;
}

Posted 15-Sep-11 11:10am

AshakiranBhatter

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Comments

Timberbird 15-Sep-11 17:22pm

Maybe the reason is that you are incrementing your vpTmp as int* and accessing values to it like char*?

[no name] 15-Sep-11 17:25pm

i am doing it because each time in a loop i want to assign character value to it and then increment the address. Since i am trying to convert the void pointer to string before beginning the loop.

Timberbird 16-Sep-11 2:19am

Yes, I can see that, but these types are of different size :). As Chuck o'Toole mentioned in his solution, int usually takes four bytes while char occupies one byte only

3 solutions

Solution 3

C#

vpTmp = ( char * ) malloc( strlen( s1 ) + 1 );
    while( s1[idx] != '\0' )
    {
        *( char * )vpTmp = s1[idx++];
        ( (int *)vpTmp )++;
    }
    ( (int * )vpTmp )++;
    *( char * )vpTmp = '\0';
    idx++;
    (int * )vpTmp = ( int * )vpTmp - idx;
    printf("The string is : %s\n", vpTmp );
    s2 = ( char * ) vpTmp;
    printf("The string is: %s\n", s2 );
    return 0;
}

Posted 16-Sep-11 2:56am

Rakesh Meel

Comments

[no name] 17-Sep-11 6:18am

thanks for your valuable suggestions but i have already implemented the using the same technique to solve the puzzle. :)

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**Malli_S** · Accepted Answer · 2011-09-15T20:28:00

Whenever you perform the pointer arithmetic operation on pointer, the result is always dependent on the pointer type (actually the size of type of the pointer). Here you're incrementing the void pointer after casting it to int pointer. Because of which the pointer will be incremented by sizeof(int) not by the sizeof(char).

Please refer to 'Pointer Arithmetic' section of this [^]article.

Have a nice pointer calculations !!!

Chuck O'Toole · Accepted Answer · 2011-09-15T19:06:00

"++" does not increment by the constant 1, it increments by the "sizeof(item)" so, if you type is as (int *) then "++" will increment to the next INTEGER. In math terms, the address contained in the pointer will be incremented by 4. Not at all what you want.

If you are treating it as a pointer to char (char *) in every use, declare it as such. Doing math with pointers is tricky, doing math on re-cast pointers is downright dangerous (oh, wait, you found that out).

Void Pointers do not give the expected result

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