Bus error with allocated memory on a heap

Question

I have Bus Error in such code:

char* mem_original;
int int_var = 987411;
mem_original = new char [250];
memcpy(&mem_original[250-sizeof(int)         


        

Answer 1

This is a common issue for developers with no experience on hardware that has alignment restrictions - such as SPARC. x86 hardware is very forgiving of misaligned access, albeit with performance impacts. Other types of hardware? SIGBUS.

This line of code:

int original_var = *((const int*)location);

invokes undefined behavior. You're taking an unsigned char * and interpreting what it points to as an int. You can't do that safely. Period. It's undefined behavior - for the very reason you're experiencing.

You're violating the strict aliasing rule. See What is the strict aliasing rule? Put simply, you can't refer to an object of one type as another type. A char * does not and can not refer to an int.

Oracle's Solaris Studio compilers actually provide a command-line argument that will let you get away with that on SPARC hardware - -xmemalign=1i (see https://docs.oracle.com/cd/E19205-01/819-5265/bjavc/index.html). Although to be fair to GCC, without that option, the forcing you do in your code will still SIGBUS under the Studio compiler.

Or, as you've already noted, you can use memcpy() to copy bytes around no matter what they are - as long as you know the source object is safe to copy into the target object - yes, there are cases when that's not true.