Programming

rvan,

Can you try with 'optimization' off in your compiler flags?(whats your OS and machine)

On most platforms, size of word is size of pointer(address) or size of register which can hold address in processor's ALU. (it is sizeof(long))

It is 4 bytes on 32 bit machine, and therefore compiler is aligning your data(variables) at 4 byte boundary or it stores variables at address multiple of 4 (divisible by 4).

This makes processor access data efficiently. Each read operation from main memory, reads a word (4 bytes), so if your data is not aligned, it needs to do more reads than it could with proper alignment

In case of array, it can not pad bytes at the end (so that next variable begins at address divisible by 4); how will it find out next element in array otherwise? Then it has to maintain then size of padding; which may be different for each array variable, as it's addresses may be different; and so whole meaning of efficiency is rendered meaningless.

Try reading on SIGBUS signal and also on structure padding/alignment.

~Thanks

The pragma directive lets you alter the packing of structures but has no affect on ordinary variables. Also it is non-portable and should not be used in code that will be deployed to multiple OSes AIX HPUX Solaris Linux. Output below is from an AIX machine...

Code:

#include <stdio.h>

#pragma pack(1)

int main(void)
{
    char beg;
    struct ptr
    {
        int a;
        char b;
        int *p;
    } abc;
    char end;

    printf("Using sizeof operator = %u\n", sizeof(abc));

    printf("beg address = %lu\n", (unsigned long) &beg);
    printf("end address = %lu\n", (unsigned long) &end);
    printf("first element address = %lu\n", (unsigned long) &abc.a);
    printf("last element address = %lu\n", (unsigned long) &abc.p);
    printf("size of structure = %lu\n", (unsigned long) &end - (unsigned long) &abc.a);
}

Output...

Code:

using sizeof operator = 9
beg address = 104399808
end address = 104399821
first element address = 104399812
last element address = 104399817
size of structure = 9

Which is a lot of architectures. And have been for the last decade, but change is finally happening.

Sure, I suppose... I guess I'm not seeing the point. If you can't do sizeof() on something, you can't measure it with pointer tricks either. Nothing's gained, and legibility is lost.

Ever played trivial pursuit?