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<h1>API Extensions</h1>
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<p>
LuaJIT is fully upwards-compatible with Lua 5.1. It supports all
<a href="http://www.lua.org/manual/5.1/manual.html#5"><span class="ext">»</span> standard Lua
library functions</a> and the full set of
<a href="http://www.lua.org/manual/5.1/manual.html#3"><span class="ext">»</span> Lua/C API
functions</a>.
</p>
<p>
LuaJIT is also fully ABI-compatible to Lua 5.1 at the linker/dynamic
loader level. This means you can compile a C module against the
standard Lua headers and load the same shared library from either Lua
or LuaJIT.
</p>
<h2 id="bit"><tt>bit.*</tt> — Bitwise Operations</h2>
<p>
LuaJIT supports all bitwise operations as defined by
<a href="http://bitop.luajit.org"><span class="ext">»</span> Lua BitOp</a>:
</p>
<pre class="code">
bit.tobit bit.tohex bit.bnot bit.band bit.bor bit.bxor
bit.lshift bit.rshift bit.arshift bit.rol bit.ror bit.bswap
</pre>
<p>
This module is a LuaJIT built-in — you don't need to download or
install Lua BitOp. The Lua BitOp site has full documentation for all
<a href="http://bitop.luajit.org/api.html"><span class="ext">»</span> Lua BitOp API functions</a>.
</p>
<p>
Please make sure to <tt>require</tt> the module before using any of
its functions:
</p>
<pre class="code">
local bit = require("bit")
</pre>
<p>
An already installed Lua BitOp module is ignored by LuaJIT.
This way you can use bit operations from both Lua and LuaJIT on a
shared installation.
</p>
<h2 id="jit"><tt>jit.*</tt> — JIT compiler control</h2>
<p>
The functions in this built-in module control the behavior
of the JIT compiler engine.
</p>
<h3 id="jit_onoff"><tt>jit.on()<br>
jit.off()</tt></h3>
<p>
Turns the whole JIT compiler on (default) or off.
</p>
<p>
These functions are typically used with the command line options
<tt>-j on</tt> or <tt>-j off</tt>.
</p>
<h3 id="jit_flush"><tt>jit.flush()</tt></h3>
<p>
Flushes the whole cache of compiled code.
</p>
<h3 id="jit_onoff_func"><tt>jit.on(func|true [,true|false])<br>
jit.off(func|true [,true|false])<br>
jit.flush(func|true [,true|false])</tt></h3>
<p>
<tt>jit.on</tt> enables JIT compilation for a Lua function (this is
the default).
</p>
<p>
<tt>jit.off</tt> disables JIT compilation for a Lua function and
flushes any already compiled code from the code cache.
</p>
<p>
<tt>jit.flush</tt> flushes the code, but doesn't affect the
enable/disable status.
</p>
<p>
The current function, i.e. the Lua function calling this library
function, can also be specified by passing <tt>true</tt> as the first
argument.
</p>
<p>
If the second argument is <tt>true</tt>, JIT compilation is also
enabled, disabled or flushed recursively for all subfunctions of a
function. With <tt>false</tt> only the subfunctions are affected.
</p>
<p>
The <tt>jit.on</tt> and <tt>jit.off</tt> functions only set a flag
which is checked when the function is about to be compiled. They do
not trigger immediate compilation.
</p>
<p>
Typical usage is <tt>jit.off(true, true)</tt> in the main chunk
of a module to turn off JIT compilation for the whole module for
debugging purposes.
</p>
<h3 id="jit_flush_tr"><tt>status = jit.flush(tr)</tt></h3>
<p>
Tries to flush the code for the specified trace and all of its
side traces from the cache. Returns <tt>true</tt> on success.
Returns <tt>false</tt> if there are still links to this trace.
</p>
<h3 id="jit_version"><tt>jit.version</tt></h3>
<p>
Contains the LuaJIT version string.
</p>
<h3 id="jit_version_num"><tt>jit.version_num</tt></h3>
<p>
Contains the version number of the LuaJIT core. Version xx.yy.zz
is represented by the decimal number xxyyzz.
</p>
<h3 id="jit_arch"><tt>jit.arch</tt></h3>
<p>
Contains the target architecture name (CPU and optional ABI).
</p>
<h2 id="jit_opt"><tt>jit.opt.*</tt> — JIT compiler optimization control</h2>
<p>
This module provides the backend for the <tt>-O</tt> command line
option.
</p>
<p>
You can also use it programmatically, e.g.:
</p>
<pre class="code">
jit.opt.start(2) -- same as -O2
jit.opt.start("-dce")
jit.opt.start("hotloop=10", "hotexit=2")
</pre>
<p>
Unlike in LuaJIT 1.x, the module is built-in and
<b>optimization is turned on by default!</b>
It's no longer necessary to run <tt>require("jit.opt").start()</tt>,
which was one of the ways to enable optimization.
</p>
<h2 id="jit_util"><tt>jit.util.*</tt> — JIT compiler introspection</h2>
<p>
This module holds functions to introspect the bytecode, generated
traces, the IR and the generated machine code. The functionality
provided by this module is still in flux and therefore undocumented.
</p>
<p>
The debug modules <tt>-jbc</tt>, <tt>-jv</tt> and <tt>-jdump</tt> make
extensive use of these functions. Please check out their source code,
if you want to know more.
</p>
<h2 id="c_api">C API extensions</h2>
<p>
LuaJIT adds some extensions to the Lua/C API. The LuaJIT include
directory must be in the compiler search path (<tt>-I<i>path</i></tt>)
to be able to include the required header for C code:
</p>
<pre class="code">
#include "luajit.h"
</pre>
<p>
Or for C++ code:
</p>
<pre class="code">
#include "lua.hpp"
</pre>
<h2 id="luaJIT_setmode"><tt>luaJIT_setmode(L, idx, mode)</tt>
— Control VM</h2>
<p>
This is a C API extension to allow control of the VM from C code. The
full prototype of <tt>LuaJIT_setmode</tt> is:
</p>
<pre class="code">
LUA_API int luaJIT_setmode(lua_State *L, int idx, int mode);
</pre>
<p>
The returned status is either success (<tt>1</tt>) or failure (<tt>0</tt>).
The second argument is either <tt>0</tt> or a stack index (similar to the
other Lua/C API functions).
</p>
<p>
The third argument specifies the mode, which is 'or'ed with a flag.
The flag can be <tt>LUAJIT_MODE_OFF</tt> to turn a feature on,
<tt>LUAJIT_MODE_ON</tt> to turn a feature off, or
<tt>LUAJIT_MODE_FLUSH</tt> to flush cached code.
</p>
<p>
The following modes are defined:
</p>
<h3 id="mode_engine"><tt>luaJIT_setmode(L, 0, LUAJIT_MODE_ENGINE|flag)</tt></h3>
<p>
Turn the whole JIT compiler on or off or flush the whole cache of compiled code.
</p>
<h3 id="mode_func"><tt>luaJIT_setmode(L, idx, LUAJIT_MODE_FUNC|flag)</tt><br>
<tt>luaJIT_setmode(L, idx, LUAJIT_MODE_ALLFUNC|flag)</tt><br>
<tt>luaJIT_setmode(L, idx, LUAJIT_MODE_ALLSUBFUNC|flag)</tt></h3>
<p>
This sets the mode for the function at the stack index <tt>idx</tt> or
the parent of the calling function (<tt>idx = 0</tt>). It either
enables JIT compilation for a function, disables it and flushes any
already compiled code or only flushes already compiled code. This
applies recursively to all subfunctions of the function with
<tt>LUAJIT_MODE_ALLFUNC</tt> or only to the subfunctions with
<tt>LUAJIT_MODE_ALLSUBFUNC</tt>.
</p>
<h3 id="mode_engine"><tt>luaJIT_setmode(L, trace,<br>
LUAJIT_MODE_TRACE|LUAJIT_MODE_FLUSH)</tt></h3>
<p>
Tries to flush the code for the specified trace and all of its
side traces from the cache.
</p>
<h3 id="mode_engine"><tt>luaJIT_setmode(L, idx, LUAJIT_MODE_WRAPCFUNC|flag)</tt></h3>
<p>
This mode defines a wrapper function for calls to C functions. The
first time this is called with <tt>LUAJIT_MODE_ON</tt>, the stack
index at <tt>idx</tt> must be a <tt>lightuserdata</tt> object holding
a pointer to the wrapper function. All <b>subsequently created C
functions</b> are called through the wrapper functions. After the initial
definition <tt>idx</tt> can be left at <tt>0</tt> when turning the mode
on or off.
</p>
<p>
The wrapper function can be used for debugging purposes or to catch
and convert foreign exceptions. Recommended usage can be seen in this
C++ code excerpt:
</p>
<pre class="code">
#include <exception>
#include "lua.hpp"
// Catch C++ exceptions and convert them to Lua error messages.
// Customize as needed for your own exception classes.
static int wrap_exceptions(lua_State *L, lua_CFunction f)
{
try {
return f(L); // Call wrapped function and return result.
} catch (const char *s) { // Catch and convert exceptions.
lua_pushstring(L, s);
} catch (std::exception& e) {
lua_pushstring(L, e.what());
} catch (...) {
lua_pushliteral(L, "caught (...)");
}
return lua_error(L); // Rethrow as a Lua error.
}
static int myregister(lua_State *L)
{
...
// Define wrapper function and enable it.
lua_pushlightuserdata(L, (void *)wrap_exceptions);
luaJIT_setmode(L, -1, LUAJIT_MODE_WRAPCFUNC|LUAJIT_MODE_ON);
lua_pop(L, 1);
luaL_register(L, "mymodule", myfuncs); // Pass luaL_Reg list.
luaJIT_setmode(L, 0, LUAJIT_MODE_WRAPCFUNC|LUAJIT_MODE_OFF);
...
// Wrap some more C++ functions which might throw an exception.
luaJIT_setmode(L, 0, LUAJIT_MODE_WRAPCFUNC|LUAJIT_MODE_ON);
lua_pushcfunction(L, mythrowingfunc1);
lua_pushcclosure(L, mythrowingfunc2, 1);
luaJIT_setmode(L, 0, LUAJIT_MODE_WRAPCFUNC|LUAJIT_MODE_OFF);
...
}
</pre>
<p>
Note that you can only define <b>a single global wrapper function</b>,
so be careful when using this mechanism from multiple C++ modules.
Also note that this mechanism is not without overhead. It should only
be enabled for definitions of C++ functions that can actually throw
exceptions. If you're embedding LuaJIT into an application, only
enable it <b>after</b> running <tt>luaL_openlibs</tt>.
</p>
<p>
LuaJIT already intercepts exception handling for systems using
DWARF2 stack unwinding (e.g. Linux, OSX). This is a zero-cost mechanism
and always enabled. You don't need to use any wrapper functions,
except when you want to get a more specific error message than
<tt>"C++ exception"</tt>.
</p>
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Copyright © 2005-2009 Mike Pall
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