# LuaTeX C API as Lua C API

I'm writing a Lua Module for LuaLaTeX.

Does a LuaTeX C API exist?

For example, how can I use the C version of tex.print into my C code?

• It is impossible to call the C version of tex.print in your C code, because the corresponding C function is marked as static and is therefore only accessible from within the same translation unit. Feb 14, 2019 at 20:50

I am by far a person that can give you an authoritative answer. As I have written in a comment to David, there is no way to use TeX as a (shared) library. But you can use LuaTeX to load modules and execute LuaTeX's functions.

Say you want to call texio.write_nl("Hello world!"). You have to write a small C program to do that:

#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <stdlib.h>
#include <stdio.h>

static int callprint (lua_State *L)
{
lua_getglobal(L, "texio");
lua_getfield(L, -1, "write_nl");
lua_remove(L, -2);
lua_pushstring(L, "Hello, world!");
lua_call(L, 1, 0);
return 0;
}

int luaopen_printfromc(lua_State *L) {
struct luaL_Reg myfuncs[] = {
{"callprint", callprint},
{NULL, NULL},
};
lua_newtable (L);
luaL_setfuncs (L, myfuncs, 0);
return 1;
}


Then you can compile this as a shared library, on my Mac it is:

cc -c  printfromc.c -I /opt/homebrew/Cellar/lua/5.2.4_1/include/
cc -flat_namespace -bundle -undefined suppress -o printfromc.so  printfromc.o


(on Debian GNU/Linux and derivatives, the following should work for TeX Live 2021:

gcc -o printfromc.so -shared -fpic  printfromc.c -I/usr/include/lua5.3/ -llua5.3


)

and copy the result to a TeX directory

cp printfromc.so /some/path/to/TeX/luatex/test


where you can load this library from the TeX source:

\documentclass{article}
\usepackage{luacode}
\begin{document}
\begin{luacode*}
x = require("printfromc")
x.callprint()

\end{luacode*}
\end{document}


In theory you can (although making work across all the platforms supported by texlive is tricky) look at the FFI module up to TeXlive 2016 that requires luajittex rather than luatex but in texlive 2017 prerelease there is some support to work with luatex.

Alternatively there is the swig library for an alternative linking mechanism.

links for ffi and swig

http://www.luatex.org/svn/trunk/source/libs/luajit/LuaJIT-src/doc/ext_ffi_semantics.html

http://www.luatex.org/swiglib.html

• You are right, of course, but I'd answer 'no' to the question. In a strict sense, there is no C-API where you can load LuaTeX as a library. One can work around it... May 21, 2017 at 10:19
• @topskip er yes perhaps I misread the direction the OP was going. Want to put up another answer? I could just delete this but the links might be useful in anycase. May 21, 2017 at 10:21
• No, I think your answer is good (+1). I made my comment just in case the OP was looking for a C-API in the sense of TeX as a library. Perhaps (unlikely) I'll give an additional one with an example. May 21, 2017 at 10:33
• @DavidCarlisle FFI will not work for this reason. Feb 14, 2019 at 23:57