Question

I have a question about structure padding and memory alignment optimizations regarding structures in C language. I am sending a structure over the network, I know that, for run-time optimizations purposes, the memory inside a structure is not contiguous. I’ve run some tests on my local computer and indeed, sizeof(my_structure) was different than the sum of all my structure members. I ran some research to find out two things :

• First, the sizeof() operator retrieves the padded size of the structure (i.e the real size that would be stored in memory).

• When specifying __attribute__((__packed__)) in the declaration of the structure this optimization is disabled by the compiler, so sizeof(my_structure) will be exactly the same as the sum of the fields of my structure.

That being said, i am wondering if the sizeof operator was getting the padded size on every compilers implementation and on every architecture, in other words, is it always safe to copy a structure with memcpy for example using the sizeof operator such as :

memcpy(struct_dest, struct_src, sizeof(struct_src)); 

I am also wondering what is the real purpose of __attribute__((__packed__)), is it used to send a less important amount the data on a network when submitting a structure or is it, in fact, used to avoid some unspecified and platform-dependant sizeof operator behaviour ?

Thanks by advance.

Answer 1

Is it always safe to copy a structure with memcpy for example using the sizeof operator

Yes, it is and that is the purpose of providing the sizeof operator.

Usually __attribute__((__packed__)) is used not for size considerations but when you want want to to make sure of the layout of a structure is exactly as you want it to be.

For ex:
If a structure is to be used to match hardware or be sent on a wire then it needs to have the exact same layout without any padding.This is because different architectures usually implement different kinds & amounts of padding and alignment and the only way to ensure common ground is to remove padding out out of the picture by using packing.