C: Creating and returning a variable in a macro

Question

I'm a bit confused as to why this C macro fails to compile:

#define LUA_GET_FIELD(Lua, idx, name, type) (\
    lua_getfield((Lua), (idx), (name)), \
    typeof(lua_to##type) __result = lua_to##type((Lua), -1), \
    lua_pop((Lua), 1), __result)

When invoking it, I get errors:

src/event.lcpp.c:134:15: error: expected primary-expression before ‘typeof’
src/event.lcpp.c:134:15: error: expected ‘)’ before ‘typeof’

The same happens if I replace typeof(lua_to##type) with just int, so I suspect the issue is to do with the commas. Can you not declare variables in the middle of an expression like this?

I know gcc has an extension for returning values from blocks, but I want to avoid compiler extensions. I also realize this should be changed to an inline function, but now I'm curious why the macro doesn't work.

Answer 1

Standard C indeed does not allow you to arbitrarily define new variables inside expressions. Older standards didn't even allow them anywhere but at the start of a block (that is, you couldn't intermix them with statements either).