Question

I am using ARM. I got alignment fault due to read/write in odd offset(we knew ARM is 4 byte aligned). All the structs defined in my program is single – byte aligned like

#pragma pack(push, 1)    

typedef struct
{
   char a1;
   int  a2;
   char a3;
}a;

#pragma pack(pop)

I am trying to do

#pragma pack(push, 1)

typedef struct
{
    char a1 __attribute__ ((aligned (4)));
    int  a2;
    char a3;
}a;

#pragma pack(pop) 

the gcc attribute _attribute_ ((aligned (4))) makes no effect.

Note :: The above code is not my actual code. sample scenario. 

so I re-arranged structure member to solve the alignment issue. I want to ensure whether the re-arranging is the possible solution or we can make _attribute_ to work on this scenario. Any other solutions are welcome. Thanks in advance.

Answer 1

You can safely read/write char/int in byte-aligned structs on ARM, compiler take care about alignment. Single problem can occur with alignment issue is with the casting to 32 bit int like this:

char buf[5];
char *p = buf + 1;
*((int *)p) = 1;

Note: if you for some reasons like align some member (from begin of the struct) you can use following trick:

typedef struct {
    struct {} __attribute__ ((aligned (4)));
    char a1; // offset is 4
    int  a2; // offset is 5
    char a3; // offset is 9
} a;