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

2.6.37IPNCDM3685.1.0R001

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.