I'm using an AT91SAM7 uC and have inherited some C code which is giving me fits. When I run this code I get a data abort error. I've tracked down the offending line of code, and it appears to be caused by something like this:
n = (UINT8) ((UINT16*)(pObject->pVar))[0];
pObject->pVar points to an instance of a structure like this:
typedef __packed struct{ UINT16 v1; UINT32 v2; UINT32 v3; UINT32 v4; UINT32 v5; }OBJTYPE; OBJTYPE xxx = {0,0,0,0,0};
When I stop the debugger on the line that causes the data abort error, I can see that pObject->pVar is pointing to 0x00200173. Is the data abort error happening because that object is not on a 32-bit boundary in memory? If so, how can I [easily] fix that? I've been going through all the online doc's reading about __packed, #pragma pack(n), adding unions to typedefs, etc.; but all the things I've been trying have not fixed the problem.
Is there an easy way (besides the __at__ attribute) to get variables like the above structure to be aligned on 32-bit boundaries?
Can anyone give me a suggestion for how to resolve this problem?
Help... Dave.
I can see three reasons for packing.
1) You need to match the memory layout of a memory-mapped device.
2) You need to conserve space.
3) You are lazy and want to access packed structures in file systems or files directly, instead of creating a #define or a helper function for reading/writing the potentially unaligned values.
Note that for 1 and 3, you may also have to take byte order into account, in which case it may not help to pack.
An ARM chip is normally fast enough that you can write code to explicitly pack/unpack data, instead of having the compiler pack the structures. That will help if moving to another compiler or processor.