Question

I have a c program running embedded Lua. as of right now, it’s just a hello world. before moving on, though, I’d like to be able to get the lua output to be sent somewhere other than stdout, so that I can manipulate it in some manner. Here’s my code:

#include <stdio.h>

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

int main() {
    lua_State *luaVM = luaL_newstate();
    //char result[1024];

    if (luaVM == NULL) {
        printf("Error initializing lua!\n");
        return -1;
    }
    luaL_openlibs(luaVM);
    luaL_dostring(luaVM, "print(\"hello world!\")");
    //Somehow put the output into result

    //printf("%s\n%s\n", result, result);

    lua_close(luaVM);
    return 0;
}

For example, I’d like to use result, seen in the comments, to print the result of the lua code twice. Can this be done?

Answer 1

This answer can get critics from some readers, but first please have a look at my blog post which I’ve prepared specially for this answer, and read the foreword why I choose this solution.

As promised, I’ve upstreamed my old Lua 5.1 output redirection patch to the latest version.
Patches are available here: 5.1.5 and 5.2.1.

Patch:

patch -p1 < ~/download/lua-5.2.1-output-redirect.patch

Build:

cd src  
make a LUA_A="liblua-5.2.1-redirect.a" SYSCFLAGS="-DLUA_USE_LINUX -DLUA_REDIRECT" SYSLIBS="-Wl,-E -ldl -lreadline -lncurses"

Check:

nm liblua-5.x.y-redirect.a | grep printf
nm liblua-5.x.y-redirect.a | grep fputs
nm liblua-5.x.y-redirect.a | grep fwrite

Test:

Obtain test program here (C/C++ mixed, sorry). Build it via:

g++ -DLUA_REDIRECT -I/path/to/lua-5.2.1/src/ -L. lua-redirect-test.cpp -llua-5.2.1-redirect -ldl -o lua-redirect-test  

Output:

===== Test 1 output =====
Lua stdout buffer:
---
hello world!

---
Lua stderr buffer:
---

---
Lua error message:
---
(null)
---
===== Test 2 output =====
Lua stdout buffer:
---

---
Lua stderr buffer:
---

---
Lua error message:
---
[string "bad_function()"]:1: attempt to call global 'bad_function' (a nil value)
---