Programming

Hi,

I'm facing BUS (invalid address alignment) issue.

Application works correctly with binaries compiled without optimization.
When "-O3" (or -O2, O1) is in use, application abort with - BUS (invalid address alignment).

The problem appears in following situation:

Code:

struct _a
{
	int _a1;
	uint64_t _a2;
}

int f2(uint64_t *obj)
{
	printf("%llu", *obj); <--- BUS ERROR
	
	return 0;
}

void f1()
{
	...
	_a* obj_a = malloc();
	
	printf("%llu", obj_a->_a2); <--- OK
	f2(&obj_a->_a2);
}

Solaris Sparc
GCC 4.2.1

Thanks

Is that really the entire code? Does it still cause that crash when minimized like that?

Thanks form the reply.
No, it's just an example. It's a big system so it's hard to present problem detailed.

For some reason, when object from struct is passed by reference to function and we want to manipulate by pointer on this object it cause BUS error.

With test function with param "struct _a *obj" (so pointer to struct is passed) works correctly.

Code:

int f3(struct _a *obj)
{	
printf("%llu", obj->_a2); <--- OK	
return 0;
}

Like I wrote, without optimization works correctly.

Optimization often causes subtle bugs from anything which uses undefined values -- things like pointing to a stack variable which went out of scope, overrunning the end of an array, etc. The crash can happen quite a distance from whatever caused it -- the corruption could've happened long ago. Code which happens to "just work" unoptimized may run far differently when the compiler starts removing in-between steps and squeezes out the empty spaces, letting things start bumping into each other.

So I'd start by logging the values fed into that function. They're probably getting corrupted somewhere. Then follow it backwards from there until you find out where the corruption is happening.

Change

Code:

struct _a
{
    int _a1;
    uint64_t _a2;
}

to

Code:

struct _a
{
    uint64_t _a2;
    int _a1;
}

Note that I swapped the order of the fields in the structure.

By defining the structure with the smaller elements first, if the structure is packed tightly, as it likely is under optimization, you're likely violating an address restriction on the value when you pass it like that.

It's unlikely that the compiler would build itself a data structure it cannot use...

GCC on SPARC?

Given that Sun's own compilers did it with an OS base utility - Solaris 10 mkfs - you'd better believe it's quite likely that GCC on SPARC will do it, too.