Bus Error Post: 10252

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Top Forums UNIX for Dummies Questions & Answers Bus Error Post 10252 by Perderabo on Saturday 10th of November 2001 04:57:39 PM

11-10-2001

Administrator Emeritus

Quote:

Originally posted by tonyt
I only know that if one trys to access a memory which in a readonly segment, the system will bumpup a Bus Error signal.

By "access" I guess you mean trying to store something there. Not to belabor the obvious, but it is legal to read a read-only segment.

There are two signals that can be delivered to a process that attempts something with an illegal memory address:

SIGBUS (ksh will say Bus error(coredump))
SIGSEGV (ksh will say Memory fault(coredump))

I've been looking through my library and I can't find any documentation that states which signal must be delivered under a specific condition. It may be that some versions of unix only use one or the other for everything. The kernel really depends on the cpu to catch these and it may be that some cpu's simply have a single trap to handle all bad addresses.

Still to me these errors have slightly different meanings, although I must admit that this is based mostly on C programming in an HP-UX environment.

SIGBUS means that you have asked to cpu to do something that is impossible. More to the point, the cpu can catch this error immediately by looking at the virtual address. There is no need to even attempt to convert the virtual address to a physical address. This is why cpu's bother to have a two-tiered check on an address. Converting a virtual address to a physical address can be a somewhat expensive operation.

SIGSEGV means that you have asked the cpu to do something that is illegal. If the kernel wanted to, it could fiddle with things to render the attempted operation legal. This error is caught after the attempted conversion of the virtual address to a physical address.

I would tend to expect that the situation tonyt is describing would result in a SIGSEGV rather than a SIGBUS....except that if you have a pointer that is both misaligned and in the wrong segment, you won't get both signals, you only get a SIGBUS. If an int is 4 bytes on your system and you try to store an int at random addresses in a read-only segment, then yes 75% of the time you will get a SIGBUS, while 25% of the time your pointer is aligned and you get the SIGSEGV.

Still tonyt may have a system that works differently than mine. And if a system can only deliver a single signal for address problems, SIGBUS would be my choice.

Perderabo

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ioping

IOPING(1)							   User Commands							 IOPING(1)

NAME

       ioping - simple disk I/O latency monitoring tool

SYNOPSYS

       ioping [-LCDRq] [-c count] [-w deadline] [-p period] [-i interval] [-s size] [-S wsize] [-o offset] device|file|directory

       ioping -h | -v

DESCRIPTION

       This tool lets you monitor I/O latency in real time.

OPTIONS

       -c count
	      Stop after count requests.

       -w deadline
	      Stop after deadline time passed.

       -p period
	      Print raw statistics for every period requests.

       -i interval
	      Set time between requests to interval (1s).

       -s size
	      Request size (4k).

       -S size
	      Working set size (1m).

       -o offset
	      Offset in input file.

       -L     Use sequential operations rather than random. This also sets request size to 256k (as in -s 256k).

       -C     Use cached I/O.

       -D     Use direct I/O.

       -R     Disk seek rate test (same as -q -i 0 -w 3 -S 64m).

       -q     Suppress human-readable output.

       -h     Display help message and exit.

       -v     Display version and exit.

   Argument suffixes
       For options that expect time argument (-i and -w), default is seconds, unless you specify one of the following suffixes (case-insensitive):

       us, usec
	      microseconds

       ms, msec
	      milliseconds

       s, sec seconds

       m, min minutes

       h, hour
	      hours

       For  options that expect "size" argument (-s, -S and -o), default is bytes, unless you specify one of the following suffixes (case-insensi-
       tive):

       s      disk sectors (a sector is always 512).

       k, kb  kilobytes

       p      memory pages (a page is always 4K).

       m, mb  megabytes

       g, gb  gigabytes

       t, tb  terabytes

       For options that expect "number" argument (-p and -c) you can optionally specify one of the following suffixes (case-insensitive):

       k      kilo (thousands, 1 000)

       m      mega (millions, 1 000 000)

       g      giga (billions, 1 000 000 000)

       t      tera (trillions, 1 000 000 000 000)

EXIT STATUS

       Returns 0 upon success. The following error codes are defined:

       1      Invalid usage (error in arguments).

       2      Error during preparation stage.

       3      Error during runtime.

EXAMPLES

       ioping .
	      Show disk I/O latency using the default values and the current directory, until interrupted.

       ioping -c 10 -s 1M /tmp
	      Measure latency on /tmp using 10 requests of 1 megabyte each.

       ioping -R /dev/sda
	      Measure disk seek rate.

       ioping -RL /dev/sda
	      Measure disk sequential speed.

SEE ALSO

       Homepage <http://code.google.com/p/ioping/>.

AUTHORS

       This program was written by Konstantin Khlebnikov <koct9i@gmail.com>.
       Man-page was written by Kir Kolyshkin <kir@openvz.org>.

								     July 2011								 IOPING(1)

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