Is it possible to align a particular structure member in single-byte aligned structure?

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.

Seem that the #pragma overrides the _ attibute . Is is possible to remove the #pragma? And use _ _attribute _ ((packed)) when necessary? — SKi
– SKi, Commented Jul 19, 2012 at 10:27
I cant remove #pragma.I m reading USB data like that. Those structs should be single-byte packed. — Jeyaram
– Jeyaram, Commented Jul 19, 2012 at 10:30
Ok, then you can set ((packed)) attribute only for thos USB structs, and remove the pragma. — SKi
– SKi, Commented Jul 19, 2012 at 10:32
The first member of a struct starts at offset 0, so it must have the same alignment as the struct itself. — Bo Persson
– Bo Persson, Commented Jul 19, 2012 at 10:32

Dmitry Poroh · Accepted Answer · 2012-07-26 21:41:05Z

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;

edited Jul 26, 2012 at 21:41

answered Jul 19, 2012 at 10:08

Dmitry Poroh

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6 Comments

Jeyaram Over a year ago

Is it possible to align a struct member to 4-byte if that struct is single-byte aligned using #pragma???

Dmitry Poroh Over a year ago

You can: try to do this: int __attribute__((aligned (4))) a2;

Jeyaram Over a year ago

Tried. no change in struct size. gives 6 only.

Jeyaram Over a year ago

Note: I want to align a1(char), not a2.

Dmitry Poroh Over a year ago

I like questions of this type.

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