No Memory Alignment with GCC

Question

I am working with some packet data. I have created structs to hold the packet data. These structs have been generated by python for a specific networking protocol.

The issue is that due to the fact that the compiler aligns the structures, when I send the data via the networking protocol, the message ends up being longer than I would like. This causes the other device to not recognize the command.

Does anyone know possible a work around this so that my packers are exactly the size the struct should be or is there a way I can turn off memory alignment?

Answer 1

In GCC, you can use __attribute__((packed)). These days GCC supports #pragma pack, too.

1. attribute documentation
2. #pragma pack documentation

Examples:

1. attribute method:

   #include <stdio.h>
   
   struct packed
   {
       char a;
       int b;
   } __attribute__((packed));
   
   struct not_packed
   {
       char a;
       int b;
   };
   
   int main(void)
   {
       printf("Packed:     %zu\n", sizeof(struct packed));
       printf("Not Packed: %zu\n", sizeof(struct not_packed));
       return 0;
   }
   

   Output:

   $ make example && ./example
   cc     example.c   -o example
   Packed:     5
   Not Packed: 8
   

2. pragma pack method:

   #include <stdio.h>
   
   #pragma pack(1)
   struct packed
   {
       char a;
       int b;
   };
   #pragma pack()
   
   struct not_packed
   {
       char a;
       int b;
   };
   
   int main(void)
   {
       printf("Packed:     %zu\n", sizeof(struct packed));
       printf("Not Packed: %zu\n", sizeof(struct not_packed));
       return 0;
   }
   

   Output:

   $ make example && ./example
   cc     example.c   -o example
   Packed:     5
   Not Packed: 8