ziglua/docs.md

Ziglua Documentation

To avoid a duplication of efforts, Ziglua does not contain full documentation on the Lua C API. Please refer to the Lua C API Documentation for full details.

This documentation provides

• An overview of Ziglua's structure and changes from the C API
• Safety considerations
• build.zig documentation
• Example code

Documentation on each individual function is found in the source code.

Moving from the C API to Zig

While efforts have been made to keep the Ziglua API similar to the C API, many changes have been made including:

• Renaming or omitting functions
• Modifying parameters (names and types) and return values
• Additional helper functions have been added

With this in mind, here are some general guidelines to help when moving from the C to Zig APIs

Naming

In general, most functions are named similarly to the original C functions. The lua_ and luaL_ prefixes have been removed, because all functions are in the Lua struct namespace. Additionally, all functions are in camelCase to match Zig naming style.

In the few cases when the auxiliary library functions have the same name as a normal C API function, the suffix Aux is added to the function name to distinguish from the normal function.

For example, the functions lua_newstate and luaL_newstate are translated to Lua.newState and Lua.newStateAux respectively.

Because Zig best practice is to communicate intent precisely, some abbreviations are expanded to make names more clear, like renaming pcall to protectedCall.

Lua Initialization

In the C API, there are two functions provided to initialize the main Lua state: lua_newstate and luaL_newstate. The former requires passing an allocator function to be used by Lua for all memory allocations, while the latter uses the default libc allocator.

Ziglua provides a third option with the Lua.init(Allocator) function, which accepts a Zig allocator. All three functions are available depending on your needs, but most likely you will want to use the Lua.init(Allocator) function. If you have special requirements for allocation, then Lua.newState would be useful. Lua.newStateAux is available if you wish to use the default libc allocator.

Safety

The Ziglua API aims to be safer than the traditional C API. That said, the way that Lua operates means that Zig cannot protect you from all errors due to the use of longjmp in C.

Here is a list of the features Ziglua uses for greater safety:

Errors

Many functions now return Zig errors rather than an integer code. The compiler will then ensure that the error is handled, or ignored. There are specific error types like ziglua.Error.Runtime for errors that have a specific meaning.

On the other hand, many functions either succeed or return an error. Rather than returning a boolean success code, these functions return the generic ziglua.Error.Fail to indicate failure. The type of failure can be determined in the context of the function called.

Booleans

Functions that return or accept C boolean integers now use the Zig bool type.

Slices

In cases where C functions use separate pointers and ints to keep track of strings, Ziglua uses a Zig slice to keep the data together.

The slices are typed to indicate the contents (zero-terminated, raw bytes, etc.)

Enums

Ziglua uses enums instead of integer codes or strings to prevent passing an invalid value to a function.

Optionals

Any value that can be NULL in the C API is marked as optional in Zig to enforce null checking.

API Differences

The major differences between the C and Zig Lua APIs are described below. This includes identifier renaming and omissions.

Continuations

All functions and types that deal with continuations have been renamed. For example, KFunction is now LuaContFn, and lua_yieldk is now yieldCont. One exception is the KContext type which has been simply renamed to Context. This is only ever used in continuation functions, so the K doesn't add much detail.

In general, just replace the "k" with the word "cont". This is just to make the API more clear and Zig-like.

lua_error and luaL_error

Because error is a reserved word in Zig, these functions have been renamed to raiseError and raiseErrorAux respectively.

string vs lstring

The "string" variant functions vs the "lstring" functions only differ by returning the length of the string. In Ziglua, the lstring functions are all named "bytes" instead. For example, lua_tolstring is Lua.toBytes. This is because these functions are typically used in cases when the string might contain zeros before the null-terminating zero.

The "string" variant functions are safe to use when the string is known to be null terminated without inner zeros.

The length of the returned string is almost always needed, so `Lua.toString() returns a zero-terminated Zig slice of the bytes with the correct length.

lua_pushvfstring

This function has been omitted because Zig does not have a va_list type, and Lua.pushFString works well enough for string formatting if variadic args are really needed.

lua_tointegerx and lua_tonumberx

Both of these functions accept an isnum return parameter to indicate if the conversion to number was successful. In the Zig version, both functions return either the number, or an error indicating the conversion was unsuccessful, and the isnum parameter is omitted.

lua_pushliteral

This is a macro for lua_pushstring, so use Lua.pushString() instead.

pcall

Both lua_pcall and lua_pcallk are expanded to protectedCall and protectedCallCont for readability.

Build Documentation

When integrating Ziglua into your projects, the following two statements are required:

1. @import() the build.zig file
2. addPackage() the Ziglua api

Note that this must be done after setting the target and build mode, otherwise Ziglua will not know that information.

const ziglua = @import("lib/ziglua/build.zig");

pub fn build(b: *Builder) void {
    ...
    exe.addPackage(ziglua.linkAndPackage(b, exe, .{}));
}

This makes the ziglua package available in your project. Access with @import("ziglua").

There are currently three options that can be passed in the third argument to ziglua.linkAndPackage():

• .use_apicheck: defaults to false. When true defines the macro LUA_USE_APICHECK in debug builds. See The C API docs for more information on this macro.

• .version: Set the Lua version to build and embed. Defaults to .lua_54. Possible values are .lua_51, .lua_52, .lua_53, and .lua_54.

• .shared: Defaults to false for embedding in a Zig program. Set to true to dynamically link the Lua source code (useful for creating shared modules).

For example, here is a ziglua.linkAndPackage() call that enables api check and embeds Lua 5.2:

exe.addPackage(ziglua.linkAndPackage(b, exe, .{ .use_apicheck = true, .version = .lua_52 }));

Examples

Here are more thorough examples that show off the Ziglua bindings in context. All examples use the previously documented build.zig setup.

Simple Lua Interpreter

This is a modified program from Programming In Lua 4th Edition

const std = @import("std");
const ziglua = @import("ziglua");

const Lua = ziglua.Lua;

pub fn main() anyerror!void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const allocator = gpa.allocator();
    defer _ = gpa.deinit();

    // Initialize The Lua vm and get a reference to the main thread
    var lua = try Lua.init(allocator);
    defer lua.deinit();

    // Open the standard libraries
    lua.openLibs();

    var stdin = std.io.getStdIn().reader();
    var stdout = std.io.getStdOut().writer();

    var buffer: [256]u8 = undefined;
    while (true) {
        _ = try stdout.write("> ");

        // Read a line of input
        const len = try stdin.read(&buffer);
        if (len == 0) break; // EOF
        if (len >= buffer.len - 1) {
            try stdout.print("error: line too long!\n", .{});
            continue;
        }

        // Ensure the buffer is null-terminated so the Lua API can read the length
        buffer[len] = 0;

        // Compile a line of Lua code
        lua.loadString(buffer[0..len :0]) catch {
            try stdout.print("{s}\n", .{lua.toString(-1) catch unreachable});
            lua.pop(1);
            continue;
        };

        // Execute a line of Lua code
        lua.protectedCall(0, 0, 0) catch {
            try stdout.print("{s}\n", .{lua.toString(-1) catch unreachable});
            lua.pop(1);
        };
    }
}

This shows a basic interpreter that reads a string from stdin. That string is parsed and compiled as Lua code and then executed.

Notice that the functions lua.loadString() and lua.protectedCall() return errors that must be handled, here printing the error message that was placed on the stack.

The lua.toString() calls are both followed with catch unreachable in this example. This function can fail if the value at the given index is not a string. The stack should contain a Lua error string, so in this example we assert that it will not fail. We also could have passed a generic error string with catch "Error"

Calling a Zig function

Registering a Zig function to be called from Lua is simple

const std = @import("std");
const ziglua = @import("ziglua");

const Lua = ziglua.Lua;

fn adder(lua: *Lua) i32 {
    const a = lua.toInteger(1) catch 0;
    const b = lua.toInteger(2) catch 0;
    lua.pushInteger(a + b);
    return 1;
}

pub fn main() anyerror!void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const allocator = gpa.allocator();
    defer _ = gpa.deinit();

    var lua = try Lua.init(allocator);
    defer lua.deinit();

    lua.pushFunction(ziglua.wrap(adder));
    lua.pushInteger(10);
    lua.pushInteger(32);

    // assert that this function call will not error
    lua.protectedCall(2, 1, 0) catch unreachable;

    std.debug.print("the result: {}\n", .{lua.toInteger(1) catch unreachable});
}

Notice the use of ziglua.wrap. This is because the function fn adder(*Lua) i32 is a ziglua.ZigFn, when the lua.pushFunction call expects a ziglua.CFn type.

The ziglua.wrap function generates a new function at compile time that wraps the Zig function in a function compatible with the Lua C API. This could be done automatically by lua.pushFunction, but that would require the parameter to be comptime-known. The call to ziglua.wrap is slightly more verbose, but has the benefit of being more flexible.