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Full Discussion: Bus Error
Top Forums UNIX for Dummies Questions & Answers Bus Error Post 10252 by Perderabo on Saturday 10th of November 2001 04:57:39 PM
Old 11-10-2001
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.
 

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LEARN ABOUT V7

phys

PHYS(2) 							System Calls Manual							   PHYS(2)

NAME

       phys - allow a process to access physical addresses

SYNOPSIS

       phys(segreg, size, physadr)

DESCRIPTION

       The  argument  segreg specifies a process virtual (data-space) address range of 8K bytes starting at virtual address segregx8K bytes.  This
       address range is mapped into physical address physadrx64 bytes.	Only the first sizex64 bytes of this mapping is addressable.  If  size	is
       zero, any previous mapping of this virtual address range is nullified.  For example, the call

	    phys(6, 1, 0177775);

       will  map  virtual  addresses  0160000-0160077  into physical addresses 017777500-017777577.  In particular, virtual address 0160060 is the
       PDP-11 console located at physical address 017777560.

       This call may only be executed by the super-user.

SEE ALSO

       PDP-11 segmentation hardware

DIAGNOSTICS

       The function value zero is returned if the physical mapping is in effect.  The value -1 is returned if not super-user, if segreg is not	in
       the range 0-7, if size is not in the range 0-127, or if the specified segreg is already used for other than a previous call to phys.

BUGS

       This  system  call is obviously very machine dependent and very dangerous.  This system call is not considered a permanent part of the sys-
       tem.

ASSEMBLER

       (phys = 52.)
       sys phys; segreg; size; physadr

								       PDP11								   PHYS(2)
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