Address alignment

2010-10-21 C address alignment

During recent work on analysis of ELF (Executable and Linkable Format) file format I needed implement proper alignment of data in sections.

How it should look like

address	binary	aligned to 4B	binary
0	0	0	00
1	1	4	100
2	10	4	100
3	11	4	100
4	100	4	100
5	101	8	1000
6	110	8	1000
7	111	8	1000
8	1000	8	1000
9	1001	12	1100
10	1010	12	1100
…	…	…	…

Note it always means that aligned address has zeroed part up-to-alignment. So the code will just add one enough to move after next aligned address and mask lower bits of result.

C implementation

inline unsigned int align(unsigned int addr, unsigned int align) {
    if(!align) align = 1;       // zero alignment means actually 1 byte
    return (addr + align-1) & ~(align-1);
}

// sample usage
new_addr = align(addr,4);