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Date: Tue, 05 Nov 2002 08:26:00 -0000
From: kyle_s_hoyt@raytheon.com
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Subject: c++/8464: Bus Error (SIGBUS) due to misaligned access
X-SW-Source: 2002-11/txt/msg00228.txt.bz2
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>Number:         8464
>Category:       c++
>Synopsis:       Bus Error (SIGBUS) due to misaligned access
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Tue Nov 05 08:26:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     Kyle Hoyt
>Release:        2.95.2 19991024
>Organization:
>Environment:
SunOS 2.6 Generic_108528-06 sun4u sparc SUNW,Ultra-250
>Description:
Copying of data from a pointer results in a SIGBUS error.
Basically, the compiler is generating a ldd instruction
when the data is not guaranteed to be 8-byte aligned.
I have tried to multiple options such as the __attribute__
((aligned)); but I can not get the compiler not to 
create the ldd instruction.
>How-To-Repeat:
Compile the included code and run.  Basically, this is
a real stripped down version of the program.  The program
receives Navigation Data messages via a data tape.  The
message is guarranteed to be 4 byte aligned but not 8 byte.
The ProcessNav routine stores a history of the navigation
data. Thus it copies the message and puts it in a FIFO.
The code works on Pentium (G++ complier) and PowerPC 
(GreenHills compiler) platforms but not on Sparc.
>Fix:
Change the code to use bcopy.
>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: text/plain; name="gccbug.txt"
Content-Disposition: inline; filename="gccbug.txt"

typedef struct
{
  int message;
  int numwords;
  double latitude;
  double longitude;
  double altitude;
  double ew_rate;
  double ns_rate;
  int time;
} NAVIGATION_DATAS __attribute__ ((aligned(4)));

void ProcessNavData(NAVIGATION_DATAS *nav_ptr)
{
  NAVIGATION_DATAS store;

  store = *nav_ptr;
}

int main()
{
  unsigned int data[1024];

  ProcessNavData((NAVIGATION_DATAS *)&data[0]);
  ProcessNavData((NAVIGATION_DATAS *)&data[1]);
}