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@tylerneylon
Created December 3, 2015 23:56
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Skeleton Lua C module for teaching.
// demo.c
//
// A skeleton Lua module written in C.
//
#include "printstack.h"
#include "lua.h"
#include "lauxlib.h"
int f(lua_State *L) {
// stack = [<args>]
print_stack(L);
return 0;
}
int g(lua_State *L) {
// stack = [<args>]
print_stack(L);
return 0;
}
#define setup_global_fn(fn_name) \
lua_pushcfunction(L, fn_name); \
lua_setglobal(L, #fn_name);
int luaopen_demo(lua_State *L) {
setup_global_fn(f);
setup_global_fn(g);
return 0;
}
all: demo.so
clean:
rm *.o *.so
demo.so: demo.o printstack.o
cc -bundle -undefined dynamic_lookup -o demo.so demo.o printstack.o
demo.o: demo.c
cc -fPIC -o $@ -c $< -Ilua_src
printstack.o: printstack.c
cc -fPIC -o $@ -c $< -Ilua_src
// printstack.c
//
#include "printstack.h"
#include "lauxlib.h"
#include <assert.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>
static void print_item(lua_State *L, int i, int as_key);
static int is_identifier(const char *s) {
while (*s) {
if (!isalnum(*s) && *s != '_') return 0;
++s;
}
return 1;
}
static int is_seq(lua_State *L, int i) {
// stack = [..]
lua_pushnil(L);
// stack = [.., nil]
int keynum = 1;
while (lua_next(L, i)) {
// stack = [.., key, value]
lua_rawgeti(L, i, keynum);
// stack = [.., key, value, t[keynum]]
if (lua_isnil(L, -1)) {
lua_pop(L, 3);
// stack = [..]
return 0;
}
lua_pop(L, 2);
// stack = [.., key]
keynum++;
}
// stack = [..]
return 1;
}
static void print_seq(lua_State *L, int i) {
printf("{");
int k;
for (k = 1;; ++k) {
// stack = [..]
lua_rawgeti(L, i, k);
// stack = [.., t[k]]
if (lua_isnil(L, -1)) break;
if (k > 1) printf(", ");
print_item(L, -1, 0); // 0 --> as_key
lua_pop(L, 1);
// stack = [..]
}
// stack = [.., nil]
lua_pop(L, 1);
// stack = [..]
printf("}");
}
static void print_table(lua_State *L, int i) {
// Ensure i is an absolute index as we'll be pushing/popping things after it.
if (i < 0) i = lua_gettop(L) + i + 1;
const char *prefix = "{";
if (is_seq(L, i)) {
print_seq(L, i); // This case includes all empty tables.
} else {
// stack = [..]
lua_pushnil(L);
// stack = [.., nil]
while (lua_next(L, i)) {
printf("%s", prefix);
// stack = [.., key, value]
print_item(L, -2, 1); // 1 --> as_key
printf(" = ");
print_item(L, -1, 0); // 0 --> as_key
lua_pop(L, 1); // So the last-used key is on top.
// stack = [.., key]
prefix = ", ";
}
// stack = [..]
printf("}");
}
}
static char *get_fn_string(lua_State *L, int i) {
static char fn_name[1024];
// Ensure i is an absolute index as we'll be pushing/popping things after it.
if (i < 0) i = lua_gettop(L) + i + 1;
// Check to see if the function has a global name.
// stack = [..]
lua_getglobal(L, "_G");
// stack = [.., _G]
lua_pushnil(L);
// stack = [.., _G, nil]
while (lua_next(L, -2)) {
// stack = [.., _G, key, value]
if (lua_rawequal(L, i, -1)) {
snprintf(fn_name, 1024, "function:%s", lua_tostring(L, -2));
lua_pop(L, 3);
// stack = [..]
return fn_name;
}
// stack = [.., _G, key, value]
lua_pop(L, 1);
// stack = [.., _G, key]
}
// If we get here, the function didn't have a global name; print a pointer.
// stack = [.., _G]
lua_pop(L, 1);
// stack = [..]
snprintf(fn_name, 1024, "function:%p", lua_topointer(L, i));
return fn_name;
}
static void print_item(lua_State *L, int i, int as_key) {
int ltype = lua_type(L, i);
// Set up first, last and start and end delimiters.
const char *first = (as_key ? "[" : "");
const char *last = (as_key ? "]" : "");
switch(ltype) {
case LUA_TNIL:
printf("nil"); // This can't be a key, so we can ignore as_key here.
return;
case LUA_TNUMBER:
printf("%s%g%s", first, lua_tonumber(L, i), last);
return;
case LUA_TBOOLEAN:
printf("%s%s%s", first, lua_toboolean(L, i) ? "true" : "false", last);
return;
case LUA_TSTRING:
{
const char *s = lua_tostring(L, i);
if (is_identifier(s) && as_key) {
printf("%s", s);
} else {
printf("%s'%s'%s", first, s, last);
}
}
return;
case LUA_TTABLE:
printf("%s", first);
print_table(L, i);
printf("%s", last);
return;
case LUA_TFUNCTION:
printf("%s%s%s", first, get_fn_string(L, i), last);
return;
case LUA_TUSERDATA:
case LUA_TLIGHTUSERDATA:
printf("%suserdata:", first);
break;
case LUA_TTHREAD:
printf("%sthread:", first);
break;
default:
printf("<internal_error_in_print_stack_item!>");
return;
}
// If we reach here, then we've got a type that we print as a pointer.
printf("%p%s", lua_topointer(L, i), last);
}
void print_stack(lua_State *L) {
int n = lua_gettop(L);
printf("stack:");
int i;
for (i = 1; i <= n; ++i) {
printf(" ");
print_item(L, i, 0); // 0 --> as_key
}
if (n == 0) printf(" <empty>");
printf("\n");
}
// printstack.h
#include "lua.h"
void print_stack(lua_State *L);
@tylerneylon
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This is a demo skeleton C module for playing around the Lua's C API. It includes a handy print_stack C function to see how you're actually affecting the stack as you go.

In order to compile this, please put the files lua.h, luaxlib.h, and luaconf.h in a directory called lua_src. Those files are included in the Lua downloads available here.

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