CODEWITHSUNDEEP
cgccmemory-alignment
joetde
joetde

Reputation: 1636

Aligning at the end of a packed struct

Is there anyway to make gcc pad only at the end of a packed structure?

I am using a packed structure for space optimization, but also use that struct to compute memory offsets (storing multiple structs in a buffer). Therefore if the total size of my struct is not aligned (not a multiple of 4 let's say), if I try to access the following struct in my buffer I will get a SIGBUS.

E.g.

sizeof(my_packed_struct) == 11
sizeof(other_struct) == 12

If I put a my_packed_struct at address 0x2000, I would also put other_struct at 0x200B (+11 bytes).

So if I want to access the other_struct, e.g. (other_struct*)0x200B, I would get a SIGBUS.

So I'm curious if there's a way to make GCC pad the structure to avoid this problem.

Here is the definition:

typedef struct __attribute__ ((packed)) my_packed_struct {
    uint8_t att1;
    bool att2;
    uint32_t att3;
    uint32_t att4;
    bool att5;
} my_packed_struct;

I could just add an attribute like

typedef struct __attribute__ ((packed)) my_packed_struct {
    uint8_t att1;
    bool att2;
    uint32_t att3;
    uint32_t att4;
    bool att5;
    uint8_t pad;
} my_packed_struct;

To make sure to match to size 12, but I'm looking for a solution where I wouldn't have to compute the size and pad manually (e.g. if I have to add another attribute in the future).

I had a look at memory alignment within gcc structs, I do store the struct in a buffer internally, but I still need to expose a struct for convenience and client use.

Upvotes: 3

Views: 594

Answers (1)

Anty
Anty

Reputation: 1516

What about this trick

union __attribute__ ((packed)) struct_union
{
    struct my_packed_struct data;
    int pad[(sizeof(my_packed_struct)+3)/sizeof(int)];
} struct_union;

and use struct_union instead of my_packed_struct directly?

Why it will work?

  1. union is packed so it will not be extra padded
  2. pad will always be multiple of 4 (because of int)
  3. sizeof calculation will make sure it is big enough to hold whole my_packed_struct

Upvotes: 1

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