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Re: unaligned trap

Thus spake Alexander Kotelnikov:
> Hello.
> Can anyone tell me, what do mean these entries in kern.log?
> Jul  9 22:38:26 vinci kernel: mplayer(3683): unaligned trap at 000000012004e258: 0000000120456b4c 29 9

Blatantly stolen from http://www.alphalinux.org/faq/FAQ-1.html#ss1.2
(which I had bookmarked after seeing similar messages with other
programs).  On some platforms unaligned accesses will cause the program
to crash, whereas on others the kernel emulates the access and lets the
program continue running.

Unaligned accesses:

    The Alpha, like all real RISC CPUs, requires that memory accesses
are naturally aligned. For example, reading a 4 byte integer from memory
requires that the address of the integer be a multiple of 4. Similarly,
8 byte integers need to start at an address that is a multiple of 8. If
the CPU attempts to access a word that is not properly aligned, the CPU
will trap into the kernel and issue a warning message. The kernel will
then go ahead and emulate the unaligned access so that the user-level
process executes as if nothing had happened (except for a substantial
slow-down due to the fault).

    Typically, an unaligned fault message looks like this:

X(26738): unaligned trap at 000000012004b6f0: 00000001401b20ca 28 1

    What this means is that the process executing command X (the X11
server) with process id 26738 caused an unaligned fault accessing
address 0x1401b20ca. This access was performed by the instruction
located at address 0x12004b6f0. The other numbers are less important,
but if you check the kernel sources, you'll find that they tell you more
info on what kind of instruction caused the fault (e.g., a load vs. a

    You do not need to be overly alarmed when seeing such a message. The
program causing the faults will work correctly. Eventually, all
unaligned accesses will be fixed, but in the meantime, just ignore these
messages (if you're a programmer, please take a minute and fix the
source of the unaligned access instead...). 

Nathan Poznick <poznick@conwaycorp.net>

The automobile has not merely taken over the street, it has dissolved
the living tissue of the city...Gas-filled, noisy and hazardous, our
streets have become the most inhumane landscape in the world. - James
M. Fitch

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