memory alignment

memory-alignment

Table of Contents

• 1. Conclusion
• 2. Test Program
• 3. Memory Alignment in ubuntu14.04 i386
  • 3.1. kernel version
  • 3.2. result
• 4. Memory Alignment in Arm DM368
  • 4.1. kernel version
  • 4.2. result

1 Conclusion

Make sure memory alignment by 4 bytes. Especially for Embedded development.

2 Test Program

#include <stdio.h>

// not alignment for 4B

int main()
{
    int i = 0;
    unsigned char buf[8];

    for (i = 0; i < 8; i++) {
        buf[i] = i;
        printf("buf[%d] = 0x%x\n", i, buf[i]);
    }

    for (i = 0; i < 5; i++)
        printf("*(int *)&buf[%d] = 0x%x\n", i, *(int *)&buf[i]);

    return 0;
}

3 Memory Alignment in ubuntu14.04 i386

 

3.1 kernel version

3.13.0-62-generic

3.2 result

buf[0] = 0x0
buf[1] = 0x1
buf[2] = 0x2
buf[3] = 0x3
buf[4] = 0x4
buf[5] = 0x5
buf[6] = 0x6
buf[7] = 0x7
*(int *)&buf[0] = 0x3020100
*(int *)&buf[1] = 0x4030201
*(int *)&buf[2] = 0x5040302
*(int *)&buf[3] = 0x6050403
*(int *)&buf[4] = 0x7060504

4 Memory Alignment in Arm DM368

 

4.1 kernel version

2.6.37_IPNCDM3685.1.0_R001

4.2 result

buf[0] = 0x0
buf[1] = 0x1
buf[2] = 0x2
buf[3] = 0x3
buf[4] = 0x4
buf[5] = 0x5
buf[6] = 0x6
buf[7] = 0x7
*(int *)&buf[0] = 0x3020100
*(int *)&buf[1] = 0x30201
*(int *)&buf[2] = 0x1000302
*(int *)&buf[3] = 0x2010003
*(int *)&buf[4] = 0x7060504

we can see when we reference 4B content from &buf¹, &buf² and &buf³, we get the wrong value.