According to the Wikipedia page Segmentation fault, a bus error can be caused by unaligned memory access. The article gives an example about how to trigger a bus error. In the example, we have to enable alignment checking to see the bus error. What if we disable such alignment checking?
The program seems to work properly. I have a program access unaligned memory frequently, and it is used by quite a few people, but no one reports bus errors or other weird results to me. If we disable alignment checking, what is the side effect of unaligned memory?
Platforms: I am working on x86/x86-64. I also tried my program by compiling it with "gcc -arch ppc" on a Mac and it works properly.
回答1:
It may be significantly slower to access unaligned memory (as in, several times slower).
Not all platforms even support unaligned access - x86 and x64 do, but ia64 (Itanium) does not, for example.
A compiler can emulate unaligned access (VC++ does that for pointers declared as __unaligned on ia64, for example) - by inserting additional checks to detect the unaligned case, and loading/storing parts of the object that straddle the alignment boundary separately. That is even slower than unaligned access on platforms which natively support it, however.
回答2:
It very much depends on the chip architecture. x86 and POWER are very forgiving, Sparc, Itanium and VAX throw different exceptions.
回答3:
Consider the following example I have just tested on ARM9:
//Addresses 0 1 2 3 4 5 6 7 8 9 U8 u8Temp[10]={0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88,0x99,0x00}; U32 u32Var; u32Var =*((U32*)(u16Temp+1));// Let's read four bytes starting from 0x22// You would expect that here u32Var will have a value of 0x55443322 (assuming we have little endian)// But in reallity u32Var will be 0x11443322!// This is because we are accessing address which %4 is not 0.
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