What is a bus error?

Question

What does the "bus error" message mean, and how does it differ from a segfault?

Answer 1

I believe the kernel raises SIGBUS when an application exhibits data misalignment on the data bus. I think that since most[?] modern compilers for most processors pad / align the data for the programmers, the alignment troubles of yore (at least) mitigated, and hence one does not see SIGBUS too often these days (AFAIK).

From: Here

Answer 2

Bus errors are rare nowadays on x86 and occur when your processor cannot even attempt the memory access requested, typically:

• using a processor instruction with an address that does not satisfy its alignment requirements.

Segmentation faults occur when accessing memory which does not belong to your process, they are very common and are typically the result of:

• using a pointer to something that was deallocated.
• using an uninitialized hence bogus pointer.
• using a null pointer.
• overflowing a buffer.

PS: To be more precise this is not manipulating the pointer itself that will cause issues, it's accessing the memory it points to (dereferencing).

Answer 3

One classic instance of a bus error is on certain architecures, such as the SPARC (at least some SPARCs, maybe this has been changed), is when you do a mis-aligned access. For instance:

unsigned char data[6];
(unsigned int *) (data + 2) = 0xdeadf00d;

This snippet tries to write the 32-bit integer value 0xdeadf00d to an address that is (most likely) not properly aligned, and will generate a bus error on architectures that are "picky" in this regard. The Intel x86 is, by the way, not such an architecture, it would allow the access (albeit execute it more slowly).