What do _2_1_ and CONCAT11 mean in Ghidra?

Question

I've been messing around with a program in Ghidra, and I came across these four lines of code. I found something about CONCAT11 meaning a concatenation between two one-byte operands, but I don't understand what part of the input user_input._2_1_ and (undefined)user_input represent.

The code in C:

  read(0,&user_input,5);
  uVar1 = user_input;
  uVar2 = user_input._1_1_;
  local_16._0_2_ = CONCAT11(user_input._2_1_,(undefined)user_input);

Answer 1

CONCAT11(x, y)

Performs ( ((uint16_t)x) << 8 ) | (uint8_t)y on bytes x and y.

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user_input._2_1_

user_input is a variable. That fact that you can use the . operator on it makes it a some kind of structure. _2_1_ would be one of its fields. Obviously, it's a generated name. I presume the two numbers are an offset and a size. So it would be a one byte field found at the third byte of the structure.

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(undefined)user_input

I don't think this is valid code.

We know that CONCAT11 will be provided a byte. So (undefined)user_input must refer to an operation that reads a byte. This byte must surely be the first byte of user_input. So that would make it a reference to user_input._0_1_.

Well, it could be the first byte of a multi-byte field (*(char*)&user_input._0_?_), but that seems unlikely.

I think (undefined) is used because of the lack of information needed to resolve this ambiguity.

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This is all guesswork.