c++ - Unaligned access causes error on ARM Cortex-M4 -

i have object has address not 4-byte aligned. causes hardfault error in cpu when there str instruction saving 2 registers.

this generated code:

   00000000 <_zn8baseareac1epcmm>:    0:   b510            push    {r4, lr}    2:   4604            mov     r4, r0    4:   6042            str     r2, [r0, #4]    6:   e9c4 3102       strd    r3, r1, [r4, #8]    a:   2001            movs    r0, #1    c:   7420            strb    r0, [r4, #16]    e:   b921            cbnz    r1, 1a <_zn8baseareac1epcmm+0x1a> 

these registers when @ line "4: 6042..."

r0   08738b82  r8          0   r1   08738bae  r9          0   r2          0  r10  082723e0   r3       2fcc  r11         0   r4   08738b82  r12         0   r5   20007630  r13  2000cb38   

as seen target register str-instructions not aligned on 4-byte. instruction str r2, [r0, #4] executed fine. hardfaults on next strd r3, r1, [r4, #8]. if manually change register r4 08738b80 not hardfault.

this c++ code generates above asm:

basearea::basearea(char * const paddress, unsigned long startoffset, unsigned long endoffset) :  m_paddress(paddress), m_start(startoffset), m_end(endoffset), m_eareatype(base_area) { 

and m_start first variable in class , has same address this (08738b82), m_end follows after on 0x08738b86.

how object aligned on 4-byte? have other solution this?

on arm-based systems cannot address 32-bit word not aligned 4-byte boundary (as error telling you). on x86 can access non-aligned data, there huge hit on performance.

example of boundary error on arm (here), tldr: storing pointer unsigned char , attempting convert double * (double pointer).

to solve problem, need request block of memory 4-byte aligned , copy non-aligned bytes + fill garbage bytes ensure 4 byte-aligned (hence perform data structure alignment manually). then, can interpret object 4-byte aligned new address.

from turboj in comments, explicit error:

cortex-m3 , m4 allow unaligned access default. not allow unalinged access strd instruction, hence fault.

you may find helpful this forcing data structure alignment on arm.