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Hooks

A hook puts a function of the mod in front of a game function. Every call the game makes to that function goes through the mod’s code first, which may change the arguments, call on, change the result, or answer alone.

Several mods may hook one function. The loader keeps a chain of links per function and detours the function once, at arming, after every mod is initialised. Turning a mod off later only rewires the chain; it never touches game code again.

Declaring a hook

Section titled “Declaring a hook”

detour! declares a static Hook: the mod’s link, with an entry point that has the game function’s signature and catches panics. The body is an ordinary function that takes a Next and the target’s arguments:

use enshwick::Next;


type MaxAltars = unsafe extern "system" fn(balancing: u64, level: u32) -> u8;


enshwick::detour! {
    static MAX_ALTARS: unsafe extern "system" fn(balancing: u64, level: u32) -> u8 = max_altars;
}


fn max_altars(next: Next<'_, MaxAltars>, balancing: u64, level: u32) -> u8 {
    // SAFETY: the rest of the chain takes the arguments the game passed.
    let vanilla = unsafe { next.call(balancing, level) };
    vanilla.max(16)
}


fn init(load: &mut enshwick::Load<'_>) -> Result<(), enshwick::Off> {
    let max_altars = load.symbol("building.max_altars")?;
    // SAFETY: the game's altar limit has the type `MAX_ALTARS` declares.
    unsafe { load.hook(&MAX_ALTARS, max_altars) }
}
  • The signature is an extern "system" fn or unsafe extern "system" fn with up to twelve arguments, named. It must match the game function exactly: the loader cannot check it.
  • next.get() returns the next link, or the game’s original when this link is the last. Call it to continue; skip it to replace the function. A call through Next costs one atomic load and one indirect call. Next is Copy.
  • load.hook(&HOOK, address) puts the link into the chain of the function at address (an absolute address from Finding the game). Hooks are linked during init only; nothing runs until arming. The call is unsafe: its SAFETY comment says why the function at address has the hook’s type, which the loader cannot check.

#[hook] writes the same declaration from the body. The static is named after the function in upper case:

use enshwick::{Next, hook};


#[hook]
fn upgrade(next: Next<'_, unsafe extern "system" fn(*mut u64)>, ctx: *mut u64) {
    // SAFETY: the original takes the same context.
    unsafe { next.call(ctx) }
}


fn init(load: &mut enshwick::Load<'_>) -> Result<(), enshwick::Off> {
    let target = load.system("upgrade_building_range")?;
    // SAFETY: the system takes the building context, as `UPGRADE` does.
    unsafe { load.hook(&UPGRADE, target) }
}

Chains and priority

Section titled “Chains and priority”

The links on one function run by priority, lower first. Links with equal priority run in load order: dependencies first, then by mod id. The default priority is 0.

use enshwick::{Next, abi::HookFlags};


enshwick::detour! {
    static LOOKUP: unsafe extern "system" fn(bases: *mut u8, npc: *const u32) -> u32 =
        lookup, priority = -10;
}


fn lookup(next: Next<'_, unsafe extern "system" fn(*mut u8, *const u32) -> u32>, bases: *mut u8, npc: *const u32) -> u32 {
    // SAFETY: forwarded unchanged.
    unsafe { next.call(bases, npc) }
}

With #[hook], the same settings go in the attribute: #[hook(priority = -10, flags = HookFlags::CALLER_FRAME)].

load.hook refuses, and returns Off:

  • a second link of the same mod on one function;
  • any link on a function another mod holds with HookFlags::EXCLUSIVE, and an exclusive link on a function another mod already links;
  • a link asked for after init.

At arming, a function the loader cannot detour (its first instructions do not decode or cannot move, or it is shorter than the 5-byte jump) stays unhooked, and every mod linked on it is turned off.

Flags

Section titled “Flags”

HookFlags has two flags, combined with HookFlags(a.0 | b.0):

  • EXCLUSIVE: no other mod may link on this function. Use it for a link that replaces the function and cannot share it. The loader links upgrade_building_range so while a mod controls altar zones.
  • CALLER_FRAME: the loader records the game caller’s stack pointer and callee-saved registers at entry of the chain. The link reads them with host().caller_frame().

Telling callers apart

Section titled “Telling callers apart”

A function that many places call often needs a change for one of them only. With CALLER_FRAME, the CallerFrame names the call site: return_address() reads the address the function returns to, which a mod pins per build in its own rows. stack() is the caller’s stack above the return address, and the callee-saved registers are as the caller had them, for reading its locals.

use std::sync::atomic::{AtomicU64, Ordering};


use enshwick::{Next, abi::HookFlags};


type NodeUnlocked = unsafe extern "system" fn(node: u32, bits: *const u64) -> u8;


/// The return address of the one call site to change; set in `init`.
static GATE: AtomicU64 = AtomicU64::new(0);


enshwick::detour! {
    static NODE_UNLOCKED: unsafe extern "system" fn(node: u32, bits: *const u64) -> u8 =
        node_unlocked, flags = HookFlags::CALLER_FRAME;
}


fn node_unlocked(next: Next<'_, NodeUnlocked>, node: u32, bits: *const u64) -> u8 {
    if node == 7 && let Some(host) = enshwick::host() {
        // SAFETY: the frame was taken at entry of this chain.
        let from = unsafe { host.caller_frame().return_address() };
        if from == GATE.load(Ordering::Relaxed) {
            return 1;
        }
    }
    // SAFETY: the rest of the chain takes the arguments the game passed.
    unsafe { next.call(node, bits) }
}


fn init(load: &mut enshwick::Load<'_>) -> Result<(), enshwick::Off> {
    GATE.store(load.symbol("my-mod.gate_return")?, Ordering::Relaxed);
    let node_unlocked = load.symbol("skill.node_unlocked")?;
    // SAFETY: the game's node unlock has the type `NODE_UNLOCKED` declares.
    unsafe { load.hook(&NODE_UNLOCKED, node_unlocked) }
}

Read the frame before calling anything hooked: it is valid until the link calls a hooked function.

The mod’s state in a hook

Section titled “The mod’s state in a hook”

declare_mod! adds instance() to the mod’s type: Some(&'static Self) once init has returned the state, None before. A hook can read settings there:

use enshwick::{Load, Mod, Next, Off};


struct MyMod {
    altars: u8,
}


enshwick::detour! {
    static MAX_ALTARS: unsafe extern "system" fn(balancing: u64, level: u32) -> u8 = max_altars;
}


fn max_altars(next: Next<'_, unsafe extern "system" fn(u64, u32) -> u8>, balancing: u64, level: u32) -> u8 {
    match MyMod::instance() {
        Some(state) => state.altars,
        // SAFETY: the original takes the arguments the game passed.
        None => unsafe { next.call(balancing, level) },
    }
}


impl Mod for MyMod {
    fn manifest() -> &'static enshwick::abi::Manifest {
        enshwick::manifest!(id: "you.my-mod", side: BOTH, join: true)
    }


    fn init(load: &mut Load<'_>) -> Result<Self, Off> {
        let max_altars = load.symbol("building.max_altars")?;
        // SAFETY: the game's altar limit has the type `MAX_ALTARS` declares.
        unsafe { load.hook(&MAX_ALTARS, max_altars) }?;
        Ok(Self { altars: 16 })
    }
}


enshwick::declare_mod!(MyMod);

State that changes while the game runs, such as settings applied by on_config_change, goes in atomics or an ArcSwap instead; see Code in the game’s frame.

Panics

Section titled “Panics”

The entry point detour! writes catches a panic in the body. The mod then reports a fault, the loader turns it off and rewires every chain past all of its links. The game runs vanilla where the mod was and never crashes for it.

The game’s function runs once at most. next.call keeps what the rest of the chain returned. When the body panics before calling it, the call goes on to the rest of the chain with the arguments the game passed. When the body panics after, the detour returns the kept value and the chain does not run again, so a destroy, a grant or a hit never happens twice.

enshwick::detour! {
    static RAISE: extern "system" fn(level: u32) -> u32 = raise;
}


fn raise(next: enshwick::Next<'_, extern "system" fn(u32) -> u32>, level: u32) -> u32 {
    let raised = next.call(level);
    // A panic here returns `raised` without running the game's function again.
    RAISED.fetch_add(raised - level, Ordering::Relaxed);
    raised
}

A panic is a last resort, not error handling. A hook that can fail returns to vanilla on its own, as enshwick-area-loot does with its OFF flag.

One function, both halves of a mod

Section titled “One function, both halves of a mod”

A client that hosts a game runs the server’s code, so a BOTH mod may link its server half and its client half in one process. When both halves need the same function, the mod still links it once: the loader refuses a second link of one mod on one function. Link one detour that answers for both, and skip the other half’s link.

Some functions serve both worlds of a hosting client, the client’s and the built-in server’s. A hook on one of them that should act for the server only asks ecs::Worlds first.

Testing

Section titled “Testing”

The test fake runs a mod’s hooks without the game: a fake loader whose chain ends at a plain function standing in for the game’s.