What are the meaning of the items in the “alignment trap” error message?

Question

During my debugging, I got the following error message.

Alignment trap: my_main (29858) PC=0x00170ad8 Instr=0xe5960008 Address=0x00f28daa FSR 0x0f3

I've looked around a bit and found a lot of information on this real-time embedded website . So now I know what PC and Instr refers to, but I still haven't found anything about the Address and FSR part.

What is FSR ? What address does Address refer to? According to proc/xxxx/maps that address is even outside the scope of my program. Does anybody know what the last two items mean? I need information on these to solve this alignment problem.

MAP output

00008000-001fe000 r-xp 00000000 03:02 16204      /home/myuser/my_main
00205000-00248000 rw-p 001f5000 03:02 16204      /home/myuser/my_main
00248000-00299000 rwxp 00248000 00:00 0
40000000-40018000 r-xp 00000000 03:01 2095       /lib/ld-2.3.3.so
40018000-4001b000 rw-p 40018000 00:00 0
4001f000-40020000 r--p 00017000 03:01 2095       /lib/ld-2.3.3.so
40020000-40021000 rw-p 00018000 03:01 2095       /lib/ld-2.3.3.so
40021000-40023000 r-xp 00000000 03:01 15724      /usr/lib/libem7.so.1.0.1
40023000-40029000 ---p 00002000 03:01 15724      /usr/lib/libem7.so.1.0.1
40029000-4002b000 rw-p 00000000 03:01 15724      /usr/lib/libem7.so.1.0.1
4002b000-40031000 r-xp 00000000 03:01 2057       /lib/tls/librt-2.3.3.so
40031000-40033000 ---p 00006000 03:01 2057       /lib/tls/librt-2.3.3.so
40033000-40038000 rw-p 00000000 03:01 2057       /lib/tls/librt-2.3.3.so
40038000-40039000 r--p 00005000 03:01 2057       /lib/tls/librt-2.3.3.so
40039000-4003a000 rw-p 00006000 03:01 2057       /lib/tls/librt-2.3.3.so
4003a000-4004a000 r-xp 00000000 03:01 2060       /lib/tls/libpthread-2.3.3.so
4004a000-40051000 rw-p 00008000 03:01 2060       /lib/tls/libpthread-2.3.3.so
40051000-40052000 r--p 0000f000 03:01 2060       /lib/tls/libpthread-2.3.3.so
40052000-40053000 rw-p 00010000 03:01 2060       /lib/tls/libpthread-2.3.3.so
40053000-40055000 rw-p 40053000 00:00 0
40055000-4010e000 r-xp 00000000 03:01 15703      /usr/lib/libstdc++.so.6.0.3
4010e000-40115000 ---p 000b9000 03:01 15703      /usr/lib/libstdc++.so.6.0.3
40115000-40119000 rw-p 000b8000 03:01 15703      /usr/lib/libstdc++.so.6.0.3
40119000-4011f000 rw-p 40119000 00:00 0
4011f000-401b7000 r-xp 00000000 03:01 2076       /lib/tls/libm-2.3.3.so
401b7000-401be000 rw-p 00090000 03:01 2076       /lib/tls/libm-2.3.3.so
401be000-401bf000 r--p 00097000 03:01 2076       /lib/tls/libm-2.3.3.so
401bf000-401c0000 rw-p 00098000 03:01 2076       /lib/tls/libm-2.3.3.so
401c0000-401c8000 r-xp 00000000 03:01 2005       /lib/libgcc_s.so.1
401c8000-401c9000 rw-p 00008000 03:01 2005       /lib/libgcc_s.so.1
401c9000-402cb000 r-xp 00000000 03:01 2078       /lib/tls/libc-2.3.3.so
402cb000-402d1000 ---p 00102000 03:01 2078       /lib/tls/libc-2.3.3.so
402d1000-402d2000 rw-p 00100000 03:01 2078       /lib/tls/libc-2.3.3.so
402d2000-402d4000 r--p 00101000 03:01 2078       /lib/tls/libc-2.3.3.so
402d4000-402d6000 rw-p 00103000 03:01 2078       /lib/tls/libc-2.3.3.so
402d6000-402d8000 rw-p 402d6000 00:00 0
402d8000-402e5000 rw-s 00000000 00:07 0          /SYSV12345678 (deleted)
402e5000-402e6000 ---p 402e5000 00:00 0
402e6000-402f5000 rwxp 402e6000 00:00 0
403c1000-4049d000 rw-p 403c1000 00:00 0
7efeb000-7f000000 rwxp 7efeb000 00:00 0

Answer 1

PC=0x00170ad8

This is telling you the current value of the program counter , it can be used to determine which instruction in your program is causing the trap. If you're on ARM as I think you are, this is actually the current instruction plus 8, so the instruction causing the trap is located at 0x00170ad0 .

Instr=0xe5960008

This is the encoding of the faulting instruction. If this is on ARM, that instruction is ldr r0, [r6, #8]

Address=0x00f28daa

This is telling you the address from which your program attempted to load, causing the fault. Assuming everything else so far is correct, this is r6 + 8 , so r6 held 0x00f28da2 at the time of the fault.

FSR 0xf3

This is the value held in the fault status register . It tells you what fault occurred. This particular value is an older encoding (pre-ARMv7) for an Alignment fault.

Answer 2

Most likely your CPU does not support reads/writes from/to memory of values at misaligned addresses. If you're accessing a 4-byte variable, then clearly Address=0x00f28daa isn't a multiple of 4. Only single bytes can be read/written at arbitrarily aligned addresses.

That article explains it well.