c - Is it possible to pack a local variable with particular alignment?

Question

code:

#include <stdio.h>
int main() {
    int a;
    printf("%p\n", &a);
    char b[10];
    printf("%p\n", &b);
    int c;
    printf("%p\n", &c);
}

outputs:

0061fefc
0061fef2
0061feec

It is obvious that b occupies 12 bytes memory becuase the default memory alignment makes the start address of c is a multiple of 4. It is possible to disable the memory alignment? I know #pragma pack can pack a structure, can it pack a local variable to make b occupy only 10 bytes? This is an interview question that makes me interested. Any help will be appreciated!

标签： cmemory-management

Answer 1

The C standard does not provide any facility to request relaxed alignment of objects, and there is little or no value in such a feature for independent objects (objects that are not members of a structure).

Structures are sometimes packed, using extensions to standard C, because the bytes in them need to be laid out consecutively for some specific use, such as transmitting them in a network message, or because padding in them may waste large amounts of space when a great many instances of the structure are held in memory. No such purpose occurs with a single independent object.

The char array shown does not occupy twelve bytes. It occupies ten bytes. The other two bytes are either unused or are used by the compiler for other purposes not evident in the addresses examined. In a structure, padding bytes are reserved—held unavailable for other use—to give the desired alignment. In this situation, bytes may have been skipped to give the desired alignment, but they are not reserved for that purpose. If a two-byte object were needed, the compiler could insert it into those bytes.

The amount of stack space wasted due to alignment requirements is generally small because, unlike with structure members, the compiler is free to rearrange objects to use space efficiently. And the stack pointer generally must still satisfy alignment requirements of the application binary interface (ABI), so some space for alignment may still be needed regardless of the alignment requirements of objects on the stack.