Strange Output

[KellyB]

#include <stdio.h>
#include <limits.h>

void printbin(void* num) {
size_t i, j;
unsigned char *pc = (unsigned char* )&num;
pc += sizeof(num) - 1;

for (i = 0 ; i < sizeof(num) ; pc--, i++ ) {
for ( j = 0; j < CHAR_BIT ; j++ ) {
putchar( *pc & ( 1 << (CHAR_BIT - j - 1) ) ? '1' : '0');
}
putchar(' ');
}
return ;
}

int main(void) {

size_t i, j;
float obj = 1234.0;
float obj1 = 1234.0;
unsigned char *pc = (unsigned char* )&obj;
pc += sizeof(obj) - 1;

for (i = 0 ; i < sizeof(obj) ; pc--, i++ ) {
for( j = 0; j < CHAR_BIT ; j++ ) {
putchar( *pc & ( 1 << (CHAR_BIT - j - 1) ) ? '1' : '0');
}
putchar(' ');
}
puts("");

printbin(&obj1);
return 0;
}

This is the output :
01000100 10011010 01000000 00000000
00000000 00011101 11111110 11100100

Why is it so ? It seems very strange.Can anyone please help me
understand whats going on here?

PS: I am using a little-endian processor

 

[Martin Ambuhl]

KellyB wrote:
[...]

void printbin(void* num) {
unsigned char *pc = (unsigned char* )&num;

^^
bogus &. num is a pointer. Remover the '&'

 

[KellyB]

That has a major problem,
which is related to the reply that you got from Martin Ambuhl.

As it stands, there is no way to determine
the size of the object that (num) points to.

sizeof(num) equals sizeof(void *).


You need:

#include <stddef.h>
void printbin(void* num, size_t size);

Aah ..Thanks Martin and Pete . Got it.

 

[Ben Bacarisse]

#include <stdio.h>
#include <limits.h>

void printbin(void* num) {
size_t i, j;
unsigned char *pc = (unsigned char* )&num;

The & is wrong. You already have the address.

pc += sizeof(num) - 1;

And the size will be wrong. The size of the print is not the size of
the object.

for (i = 0 ; i < sizeof(num) ; pc--, i++ ) {
for ( j = 0; j < CHAR_BIT ; j++ ) {
putchar( *pc & ( 1 << (CHAR_BIT - j - 1) ) ? '1' : '0');
}
putchar(' ');
}
return ;
}

int main(void) {

size_t i, j;
float obj = 1234.0;
float obj1 = 1234.0;
unsigned char *pc = (unsigned char* )&obj;
pc += sizeof(obj) - 1;

Both OK here, though.

for (i = 0 ; i < sizeof(obj) ; pc--, i++ ) {

There is a subtlety about running pointer loops backwards in that you
can't even construct a pointer that points "before" the first byte of
the object (you don't access it, but even constructing it is
technically undefined behaviour). You won't, in practise, experience
a problem though.