newcclosure

Creates a new C closureClosureA function value that captures its lexical environment. In Luau, closures come in two types: LClosure (Luau bytecode + Proto + upvalues) and CClosure (native C function pointer + upvalues). (CClosure) that wraps the provided Luau function. When called, the C closure internally invokes the original Luau function via lua_call, passing through all arguments and return values. The wrapper is indistinguishable from a native C function to introspection.

Luau has two closureClosureA function value that captures its lexical environment. In Luau, closures come in two types: LClosure (Luau bytecode + Proto + upvalues) and CClosure (native C function pointer + upvalues). representations sharing a common ClosureHeader:

// Common header for both types
struct ClosureHeader {
  GCObject header;     // GC linked list + type tag
  uint8_t isC;         // 0 = LClosure, 1 = CClosure
  uint8_t nupvalues;   // number of upvalues
  Table* env;          // environment table
};

// Luau closure (isC=0)
typedef struct LClosure {
  ClosureHeader;
  Proto* p;            // bytecode + constants
  UpVal* uprefs[];     // captured variables
} LClosure;

// C closure (isC=1)
typedef struct CClosure {
  ClosureHeader;
  lua_CFunction f;     // native function pointer
  TValue upvals[];     // embedded upvalue storage
} CClosure;

newcclosure(fn) allocates a CClosure with 1 upvalue (the original LClosure), and sets f to a trampolineTrampolineA wrapper function that bounces (redirects) calls to another function. In executor context, newcclosure creates a C closure trampoline that calls the Luau function stored in its first upvalue. that calls lua_call on the upvalue. Introspection sees isC=1.

Roblox's anti-cheatAnti-cheatGame-side Luau scripts designed to detect and prevent exploitation. Common checks: verifying closure types (iscclosure), checking metatable integrity, monitoring remote call patterns, and scanning for known executor globals. and hook-detection code checks the isC byte on closures. Game metamethodsMetamethodA function in a metatable that overrides default behavior for operations like indexing (__index), assignment (__newindex), calling (__call), comparison (__eq), and arithmetic (__add, __mul, etc.). like __namecall and __index are always C closures — replacing them with a Luau closureClosureA function value that captures its lexical environment. In Luau, closures come in two types: LClosure (Luau bytecode + Proto + upvalues) and CClosure (native C function pointer + upvalues). flips isC from 1 to 0, immediately detectable. newcclosure disguises your Luau hook to keep isC=1. Checks like debug.info(f, "s") == "[C]" and iscclosure(f) both pass.