Is serialization of a c++ struct a must if you send or receive them in message queue?

Question

In the legacy code, I found many struct are sent or received through system V message queue without any serialization or deserialization. The data member of a struct might be also a struct. The depth can be three. But all the data members of these struct are types of int, float, char or their corresponding arrays. A typical snippet is like this:

A a;  //A is a struct or a struct with struct data members.
....  //initialize a
msgsnd(msgid, &a, sizeof(A), IPC_NOWAIT);

Initially, I thought the serialization is needed. However, I changed my mind later. Although there are paddings for alignment of data members in struct, sending a struct like this without serialization won't cause any problem. Because there are no pointer data members. Through my simple testing, I didn't see any problem. Can anybody confirm that serialization is not needed in this circumstance? Thanks!

Is this within the same process, like to another thread, or between processes? Are there any pointers in the data? Are there any endian concerns? — tadman, May 30 '23 at 20:20
Don't forget about undefined behavior which means anything can happen including but not limited to the program giving your expected oitput. — Jason, May 30 '23 at 20:21
C++ has "trivially copyable" types that can be (de)serialized like this. It also has "standard layout" types that are expected to have more or less predictable layout between C++ and C at the least. Combining the two, you get "Plain Old Data" (POD) types. Probably you have one of these. You should be fine as long as you don't use any pointers, padding rules are the same for all users (that's not covered by any standard), compiler's extensions are the same, endianness is the same, and sizes and representations of all types are the same. — yeputons, May 30 '23 at 20:25
It can between threads of a process or between two processes on the same machine. They all expect the same endianness. — Wqh, May 30 '23 at 20:33
No, serialization not required. You can create a character buffer, and print the members to the buffer, in human readable form. The receiver can convert from human readable to internal format. This is advantageous in many uses, including debugging. — Thomas Matthews, May 30 '23 at 20:40
@ThomasMatthews Well, I'd consider that one possible way of serialisation... — Aconcagua, May 30 '23 at 20:50
*Can anybody confirm that serialization is not needed in this circumstance?* -- You can confirm this: `#include ... std::cout << std::is_trivially_copyable();` and see if you get a 1 or a 0 printed. If it's 0, then serialization is required. — PaulMcKenzie, May 30 '23 at 20:57
Or: `static_assert(std::is_trivially_copyable(), "A struct requires serialization");` and see if the compilation stops due to the `static_assert` firing. Also, if someone other than yourself (or maybe even you) inadvertantly or on purpose adds members to this struct that makes the struct no longer trivially copyable, the `static_assert` will stop the compilation. — PaulMcKenzie, May 30 '23 at 21:03

score 2 · Answer 1 · answered May 31 '23 at 03:06

Can anybody confirm that serialization is not needed in this circumstance?

If your struct does not contain pointers, then an additional check can be done by issuing the following:

#include <type_traits>
//...
struct A 
{
  //... stuff
};

void some_function()
{
   static_assert(std::is_trivially_copyable<A>(), "The A struct must be serialized and is not trivially-copyable");
}

This will stop the compilation if the compiler detects that A is not trivially-copyable. This also prevents any future changes to A from breaking your code.

If you were to add a pointer member to the struct, then the compile-time check becomes trickier. Then it would come down to visual inspection, or possibly using magic_get, as mentioned in this answer.

Is serialization of a c++ struct a must if you send or receive them in message queue?

1 Answers1