not understanding folowing C CODE...

[suma_ranga]

hi all,

i have a doubt in the following c code..the output of the code is 20 and 276... can anyone explain y this is so. what is happening in each step. y (int)n = 276 instead of 256..

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

int main(int argc, char *argv[])
{
int n = 256;
int *p = &n;
char *pp = (char*)p;
*pp = 20;


pp= printf("%d", (int)*pp);
n =printf("%d", (int)n);
system("PAUSE");
return 0;
}

 

[enjunear]

When N is defined as an integer, it is made up of 16 bits. 256 is (0000 0001 0000 0000) in binary.

char *p refers to the first 8 bits starting at the beginning memory location of N, (0000 0001). When you define *pp as (char *)p, you are pointing to the next 8 bits AFTER *p, so it points to the last 8 bits of N, which are (0000 0000).

When you write 20 (0001 0100) to *pp, you overwrite the lower 8 bits of n, so the whole memory space now looks like (0000 0001 0001 0100). The top 8 bits are untouched (which is the only bit needed to represent 256), but the lower 8 are now the bit pattern for 20. So, if you put them all together, you have bits enabled for 256, 16, and 4... sum them up (if you interpret n as a 16-bit interger), you get 276.

 

[suma_ranga]

Hi buddy,

Thank u so much.. very clearly u have explained. thnk u....

When N is defined as an integer, it is made up of 16 bits. 256 is (0000 0001 0000 0000) in binary.

char *p refers to the first 8 bits starting at the beginning memory location of N, (0000 0001). When you define *pp as (char *)p, you are pointing to the next 8 bits AFTER *p, so it points to the last 8 bits of N, which are (0000 0000).

When you write 20 (0001 0100) to *pp, you overwrite the lower 8 bits of n, so the whole memory space now looks like (0000 0001 0001 0100). The top 8 bits are untouched (which is the only bit needed to represent 256), but the lower 8 are now the bit pattern for 20. So, if you put them all together, you have bits enabled for 256, 16, and 4... sum them up (if you interpret n as a 16-bit interger), you get 276.