跟着 Lua 5.1 官方参考文档学习 Lua (7)

3 – The Application Program Interface

This section describes the C API for Lua, that is, the set of C functions available to the host program to communicate with Lua. All API functions and related types and constants are declared in the header file lua.h.

Even when we use the term “function”, any facility in the API may be provided as a macro instead. All such macros use each of their arguments exactly once (except for the first argument, which is always a Lua state), and so do not generate any hidden side-effects.

As in most C libraries, the Lua API functions do not check their arguments for validity or consistency. However, you can change this behavior by compiling Lua with a proper definition for the macro luai_apicheck, in file luaconf.h.

3.1 – The Stack

Lua uses a virtual stack to pass values to and from C. Each element in this stack represents a Lua value (nil, number, string, etc.).

Whenever Lua calls C, the called function gets a new stack, which is independent of previous stacks and of stacks of C functions that are still active. This stack initially contains any arguments to the C function and it is where the C function pushes its results to be returned to the caller (see lua_CFunction).

For convenience, most query operations in the API do not follow a strict stack discipline. Instead, they can refer to any element in the stack by using an index: A positive index represents an absolute stack position (starting at 1); a negative index represents an offset relative to the top of the stack. More specifically, if the stack has n elements, then index 1 represents the first element (that is, the element that was pushed onto the stack first) and index n represents the last element; index -1 also represents the last element (that is, the element at the top) and index -n represents the first element. We say that an index is valid if it lies between 1 and the stack top (that is, if 1 ≤ abs(index) ≤ top).

Top (栈顶)
  |
  V
+---+
| B | <- 2  -1
+---+
| A | <- 1  -2
，最先入栈的元素
+---+
  |
栈底

记住最后一个入栈的元素的索引是-1。

3.2 – Stack Size

When you interact with Lua API, you are responsible for ensuring consistency. In particular, you are responsible for controlling stack overflow. You can use the function lua_checkstack to grow the stack size.

Whenever Lua calls C, it ensures that at least LUA_MINSTACK stack positions are available. LUA_MINSTACK is defined as 20, so that usually you do not have to worry about stack space unless your code has loops pushing elements onto the stack.

Most query functions accept as indices any value inside the available stack space, that is, indices up to the maximum stack size you have set through lua_checkstack. Such indices are called acceptable indices. More formally, we define an acceptable index as follows:

     (index < 0 && abs(index) <= top) ||
     (index > 0 && index <= stackspace)

Note that 0 is never an acceptable index.

3.3 – Pseudo-Indices

Unless otherwise noted, any function that accepts valid indices can also be called with pseudo-indices, which represent some Lua values that are accessible to C code but which are not in the stack. Pseudo-indices are used to access the thread environment, the function environment, the registry, and the upvalues of a C function (see §3.4).

The thread environment (where global variables live) is always at pseudo-index LUA_GLOBALSINDEX. The environment of the running C function is always at pseudo-index LUA_ENVIRONINDEX.

To access and change the value of global variables, you can use regular table operations over an environment table. For instance, to access the value of a global variable, do

     lua_getfield(L, LUA_GLOBALSINDEX, varname);

例子：获取 Lua 中全局变量的值

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

int main() {
    lua_State *L = luaL_newstate();  // 创建一个新的 Lua 状态
    luaL_openlibs(L);  // 打开 Lua 标准库

    // 执行 Lua 代码定义一个变量
    const char *lua_code = "x = 42";
    if (luaL_dostring(L, lua_code) != LUA_OK) {
        printf("Error: %s\n", lua_tostring(L, -1));
        return 1;
    }

    // 获取 Lua 中的变量 x
    // #define lua_getglobal(L,s)  lua_getfield(L, LUA_GLOBALSINDEX, s)
    lua_getglobal(L, "x");  // 将 Lua 全局变量 x 压入栈

    // 检查栈顶的数据类型并获取其值
    if (lua_isnumber(L, -1)) {
        double x = lua_tonumber(L, -1);
        printf("Value of x: %f\n", x);  // 输出 x 的值
    }

    lua_pop(L, 1);  // 弹出栈顶的 x 变量

    lua_close(L);  // 关闭 Lua 状态
    return 0;
}

3.4 – C Closures

When a C function is created, it is possible to associate some values with it, thus creating a C closure; these values are called upvalues and are accessible to the function whenever it is called (see lua_pushcclosure).

Whenever a C function is called, its upvalues are located at specific pseudo-indices. These pseudo-indices are produced by the macro lua_upvalueindex. The first value associated with a function is at position lua_upvalueindex(1), and so on. Any access to lua_upvalueindex(*n*), where n is greater than the number of upvalues of the current function (but not greater than 256), produces an acceptable (but invalid) index.

例子：C 函数闭包实现计数器

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

// 定义一个 C 函数
int counter(lua_State *L) {
    // 获取 upvalue 的值
    int upvalue1 = lua_tointeger(L, lua_upvalueindex(1));  
    upvalue1 = upvalue1 + 1;
    // 更新 upvalue 的值
    lua_pushinteger(L, upvalue1);
    lua_replace(L, lua_upvalueindex(1));
    // 将返回值压栈
    lua_pushnumber(L, upvalue1);
    return 1;  // 返回 1 个值
}

int main() {
    lua_State *L = luaL_newstate();  // 创建 Lua 状态
    luaL_openlibs(L);  // 打开 Lua 标准库

    // 注册 C 函数到 Lua
    lua_pushinteger(L, 0);  // 压入 upvalue
    lua_pushcclosure(L, counter, 1);  // 创建闭包并将1个 upvalue 绑定到闭包上

    // 将这个闭包注册到全局变量中
    lua_setglobal(L, "counter");

    if (luaL_dostring(L, "for i = 1,3 do print(counter()) end") != LUA_OK) {
        printf("Error: %s\n", lua_tostring(L, -1));
        return 1;
    }

    lua_close(L);  // 关闭 Lua 状态
    return 0;
}

输出

1
2
3

3.5 – Registry

Lua provides a registry, a pre-defined table that can be used by any C code to store whatever Lua value it needs to store. This table is always located at pseudo-index LUA_REGISTRYINDEX. Any C library can store data into this table, but it should take care to choose keys different from those used by other libraries, to avoid collisions. Typically, you should use as key a string containing your library name or a light userdata with the address of a C object in your code.

The integer keys in the registry are used by the reference mechanism, implemented by the auxiliary library, and therefore should not be used for other purposes.

例子：向注册表中添加数据

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


int main() {
    lua_State *L = luaL_newstate();  // 创建 Lua 状态

    // 创建一个新的表并将其作为数据存储
    lua_newtable(L);  // 在栈上创建一个新的表
    
    // 向表中添加数据
    lua_pushstring(L, "name");
    lua_pushstring(L, "Lua C Example");
    lua_settable(L, -3);  // 将键值对 ("name", "Lua C Example") 添加到表中
    
    // 获取注册表
    lua_pushvalue(L, LUA_REGISTRYINDEX);  // 压入注册表的值
    lua_pushstring(L, "my_key");  // 使用独特的字符串作为键
    lua_pushvalue(L, -3);  // 将table的值复制，压入栈顶
    lua_settable(L, -3);  // 将table存储到注册表中，key为"my_key"

    // 恢复栈
    lua_pop(L, 2);

    lua_close(L);  // 关闭 Lua 状态
    return 0;
}

3.6 – Error Handling in C

Internally, Lua uses the C longjmp facility to handle errors. (You can also choose to use exceptions if you use C++; see file luaconf.h.) When Lua faces any error (such as memory allocation errors, type errors, syntax errors, and runtime errors) it raises an error; that is, it does a long jump. A protected environment uses setjmp to set a recover point; any error jumps to the most recent active recover point.

Most functions in the API can throw an error, for instance due to a memory allocation error. The documentation for each function indicates whether it can throw errors.

Inside a C function you can throw an error by calling lua_error.

例子：C 函数抛出错误

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

static int my_function(lua_State *L) {
    // 获取传入的参数
    const char *arg = lua_tostring(L, 1);  // 获取第一个参数（假设它是字符串）

    // 检查参数是否为 "expected_value"，如果不是，抛出一个错误
    if (arg == NULL || strcmp(arg, "expected_value") != 0) {
        // 使用 lua_error 抛出错误
        lua_pushstring(L, "Error: invalid argument. Expected 'expected_value'.");
        return lua_error(L);  // 抛出错误
    }

    // 如果没有错误，返回一个结果
    lua_pushstring(L, "Success! Argument is valid.");
    return 1;  // 返回一个结果
}

int main() {
    lua_State *L = luaL_newstate();  // 创建 Lua 状态
    luaL_openlibs(L);  // 打开 Lua 标准库

    // 注册函数到 Lua 中
    lua_register(L, "my_function", my_function);

    if (luaL_dostring(L, "print(my_function('invalid_value'))") != LUA_OK) {
        printf("Error: %s\n", lua_tostring(L, -1));
        return 1;
    }

    lua_close(L);  // 关闭 Lua 状态
    return 0;
}

输出

Error: Error: invalid argument. Expected 'expected_value'.

3.7 – Functions and Types

Here we list all functions and types from the C API in alphabetical order. Each function has an indicator like this: [-o, +p, x]

The first field, o, is how many elements the function pops from the stack. The second field, p, is how many elements the function pushes onto the stack. (Any function always pushes its results after popping its arguments.) A field in the form x|y means the function can push (or pop) x or y elements, depending on the situation; an interrogation mark ‘?’ means that we cannot know how many elements the function pops/pushes by looking only at its arguments (e.g., they may depend on what is on the stack). The third field, x, tells whether the function may throw errors: ‘-’ means the function never throws any error; ‘m’ means the function may throw an error only due to not enough memory; ‘e’ means the function may throw other kinds of errors; ‘v’ means the function may throw an error on purpose.

内存分配

lua_Alloc

typedef void * (*lua_Alloc) (void *ud,
                             void *ptr,
                             size_t osize,
                             size_t nsize);

The type of the memory-allocation function used by Lua states. The allocator function must provide a functionality similar to realloc, but not exactly the same. Its arguments are ud, an opaque pointer passed to lua_newstate; ptr, a pointer to the block being allocated/reallocated/freed; osize, the original size of the block; nsize, the new size of the block. ptr is NULL if and only if osize is zero. When nsize is zero, the allocator must return NULL; if osize is not zero, it should free the block pointed to by ptr. When nsize is not zero, the allocator returns NULL if and only if it cannot fill the request. When nsize is not zero and osize is zero, the allocator should behave like malloc. When nsize and osize are not zero, the allocator behaves like realloc. Lua assumes that the allocator never fails when osize >= nsize.

Here is a simple implementation for the allocator function. It is used in the auxiliary library by luaL_newstate.

     static void *l_alloc (void *ud, void *ptr, size_t osize,
                                                size_t nsize) {
       (void)ud;  (void)osize;  /* not used */
       if (nsize == 0) {
         free(ptr);
         return NULL;
       }
       else
         return realloc(ptr, nsize);
     }

This code assumes that free(NULL) has no effect and that realloc(NULL, size) is equivalent to malloc(size). ANSI C ensures both behaviors

lua_getallocf

[-0, +0, -]

lua_Alloc lua_getallocf (lua_State *L, void **ud);

Returns the memory-allocation function of a given state. If ud is not NULL, Lua stores in *ud the opaque pointer passed to lua_newstate

lua_setallocf

[-0, +0, -]

void lua_setallocf (lua_State *L, lua_Alloc f, void *ud);

Changes the allocator function of a given state to f with user data ud.

Lua 解释器

lua_State

typedef struct lua_State lua_State;

Opaque structure that keeps the whole state of a Lua interpreter. The Lua library is fully reentrant: it has no global variables. All information about a state is kept in this structure.

A pointer to this state must be passed as the first argument to every function in the library, except to lua_newstate, which creates a Lua state from scratch.

lua_newstate

[-0, +0, -]

lua_State *lua_newstate (lua_Alloc f, void *ud);

Creates a new, independent state. Returns NULL if cannot create the state (due to lack of memory). The argument f is the allocator function; Lua does all memory allocation for this state through this function. The second argument, ud, is an opaque pointer that Lua simply passes to the allocator in every call.

lua_close

[-0, +0, -]

void lua_close (lua_State *L);

Destroys all objects in the given Lua state (calling the corresponding garbage-collection metamethods, if any) and frees all dynamic memory used by this state. On several platforms, you may not need to call this function, because all resources are naturally released when the host program ends. On the other hand, long-running programs, such as a daemon or a web server, might need to release states as soon as they are not needed, to avoid growing too large.

栈操作

lua_gettop

[-0, +0, -]

int lua_gettop (lua_State *L);

Returns the index of the top element in the stack. Because indices start at 1, this result is equal to the number of elements in the stack (and so 0 means an empty stack).

lua_settop

[-?, +?, -]

void lua_settop (lua_State *L, int index);

Accepts any acceptable index, or 0, and sets the stack top to this index. If the new top is larger than the old one, then the new elements are filled with nil. If index is 0, then all stack elements are removed.

lua_pop

[-n, +0, -]

void lua_pop (lua_State *L, int n);

Pops n elements from the stack.

lua_checkstack

[-0, +0, m]

int lua_checkstack (lua_State *L, int extra);

Ensures that there are at least extra free stack slots in the stack. It returns false if it cannot grow the stack to that size. This function never shrinks the stack; if the stack is already larger than the new size, it is left unchanged.

lua_pushvalue

[-0, +1, -]

void lua_pushvalue (lua_State *L, int index);

Pushes a copy of the element at the given valid index onto the stack.

lua_remove

[-1, +0, -]

void lua_remove (lua_State *L, int index);

Removes the element at the given valid index, shifting down the elements above this index to fill the gap. Cannot be called with a pseudo-index, because a pseudo-index is not an actual stack position.

lua_insert

[-1, +1, -]

void lua_insert (lua_State *L, int index);

Moves the top element into the given valid index, shifting up the elements above this index to open space. Cannot be called with a pseudo-index, because a pseudo-index is not an actual stack position.

lua_replace

[-1, +0, -]

void lua_replace (lua_State *L, int index);

Moves the top element into the given position (and pops it), without shifting any element (therefore replacing the value at the given position).

lua_xmove

[-?, +?, -]

void lua_xmove (lua_State *from, lua_State *to, int n);

Exchange values between different threads of the same global state.

This function pops n values from the stack from, and pushes them onto the stack to.

lua_isnone

[-0, +0, -]

int lua_isnone (lua_State *L, int index);

Returns 1 if the given acceptable index is not valid (that is, it refers to an element outside the current stack), and 0 otherwise.

创建不同类型的值

lua_pushnil

[-0, +1, -]

void lua_pushnil (lua_State *L);

Pushes a nil value onto the stack.

lua_pushboolean

[-0, +1, -]

void lua_pushboolean (lua_State *L, int b);

Pushes a boolean value with value b onto the stack.

lua_Integer

typedef ptrdiff_t lua_Integer;

The type used by the Lua API to represent integral values.

By default it is a ptrdiff_t, which is usually the largest signed integral type the machine handles “comfortably”.

lua_pushinteger

[-0, +1, -]

void lua_pushinteger (lua_State *L, lua_Integer n);

Pushes a number with value n onto the stack.

lua_Number

typedef double lua_Number;

The type of numbers in Lua. By default, it is double, but that can be changed in luaconf.h.

Through the configuration file you can change Lua to operate with another type for numbers (e.g., float or long).

lua_pushnumber

[-0, +1, -]

void lua_pushnumber (lua_State *L, lua_Number n);

Pushes a number with value n onto the stack.

lua_pushstring

[-0, +1, m]

void lua_pushstring (lua_State *L, const char *s);

Pushes the zero-terminated string pointed to by s onto the stack. Lua makes (or reuses) an internal copy of the given string, so the memory at s can be freed or reused immediately after the function returns. The string cannot contain embedded zeros; it is assumed to end at the first zero.

lua_pushlstring

[-0, +1, m]

void lua_pushlstring (lua_State *L, const char *s, size_t len);

Pushes the string pointed to by s with size len onto the stack. Lua makes (or reuses) an internal copy of the given string, so the memory at s can be freed or reused immediately after the function returns. The string can contain embedded zeros.

lua_pushliteral

[-0, +1, m]

void lua_pushliteral (lua_State *L, const char *s);

This macro is equivalent to lua_pushlstring, but can be used only when s is a literal string. In these cases, it automatically provides the string length.

lua_pushfstring

[-0, +1, m]

const char *lua_pushfstring (lua_State *L, const char *fmt, ...);

Pushes onto the stack a formatted string and returns a pointer to this string. It is similar to the C function sprintf, but has some important differences:

• You do not have to allocate space for the result: the result is a Lua string and Lua takes care of memory allocation (and deallocation, through garbage collection).
• The conversion specifiers are quite restricted. There are no flags, widths, or precisions. The conversion specifiers can only be ‘%%’ (inserts a ‘%’ in the string), ‘%s’ (inserts a zero-terminated string, with no size restrictions), ‘%f’ (inserts a lua_Number), ‘%p’ (inserts a pointer as a hexadecimal numeral), ‘%d’ (inserts an int), and ‘%c’ (inserts an int as a character).

lua_pushvfstring

[-0, +1, m]

const char *lua_pushvfstring (lua_State *L,
                              const char *fmt,
                              va_list argp);

Equivalent to lua_pushfstring, except that it receives a va_list instead of a variable number of arguments.

lua_createtable

[-0, +1, m]

void lua_createtable (lua_State *L, int narr, int nrec);

Creates a new empty table and pushes it onto the stack. The new table has space pre-allocated for narr array elements and nrec non-array elements. This pre-allocation is useful when you know exactly how many elements the table will have. Otherwise you can use the function lua_newtable.

lua_newtable

[-0, +1, m]

void lua_newtable (lua_State *L);

Creates a new empty table and pushes it onto the stack. It is equivalent to lua_createtable(L, 0, 0).

lua_CFunction

typedef int (*lua_CFunction) (lua_State *L);

Type for C functions.

In order to communicate properly with Lua, a C function must use the following protocol, which defines the way parameters and results are passed: a C function receives its arguments from Lua in its stack in direct order (the first argument is pushed first). So, when the function starts, lua_gettop(L) returns the number of arguments received by the function. The first argument (if any) is at index 1 and its last argument is at index lua_gettop(L). To return values to Lua, a C function just pushes them onto the stack, in direct order (the first result is pushed first), and returns the number of results. Any other value in the stack below the results will be properly discarded by Lua. Like a Lua function, a C function called by Lua can also return many results.

As an example, the following function receives a variable number of numerical arguments and returns their average and sum:

     static int foo (lua_State *L) {
       int n = lua_gettop(L);    /* number of arguments */
       lua_Number sum = 0;
       int i;
       for (i = 1; i <= n; i++) {
         if (!lua_isnumber(L, i)) {
           lua_pushstring(L, "incorrect argument");
           lua_error(L);
         }
         sum += lua_tonumber(L, i);
       }
       lua_pushnumber(L, sum/n);        /* first result */
       lua_pushnumber(L, sum);         /* second result */
       return 2;                   /* number of results */
     }

lua_pushcclosure

[-n, +1, m]

void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);

Pushes a new C closure onto the stack.

When a C function is created, it is possible to associate some values with it, thus creating a C closure (see §3.4); these values are then accessible to the function whenever it is called. To associate values with a C function, first these values should be pushed onto the stack (when there are multiple values, the first value is pushed first). Then lua_pushcclosure is called to create and push the C function onto the stack, with the argument n telling how many values should be associated with the function. lua_pushcclosure also pops these values from the stack.

The maximum value for n is 255.

lua_pushcfunction

[-0, +1, m]

void lua_pushcfunction (lua_State *L, lua_CFunction f);

Pushes a C function onto the stack. This function receives a pointer to a C function and pushes onto the stack a Lua value of type function that, when called, invokes the corresponding C function.

Any function to be registered in Lua must follow the correct protocol to receive its parameters and return its results (see lua_CFunction).

lua_pushcfunction is defined as a macro:

     #define lua_pushcfunction(L,f)  lua_pushcclosure(L,f,0)

lua_pushlightuserdata

[-0, +1, -]

void lua_pushlightuserdata (lua_State *L, void *p);

Pushes a light userdata onto the stack.

Userdata represent C values in Lua. A light userdata represents a pointer. It is a value (like a number): you do not create it, it has no individual metatable, and it is not collected (as it was never created). A light userdata is equal to “any” light userdata with the same C address.

lua_newuserdata

[-0, +1, m]

void *lua_newuserdata (lua_State *L, size_t size);

This function allocates a new block of memory with the given size, pushes onto the stack a new full userdata with the block address, and returns this address.

Userdata represent C values in Lua. A full userdata represents a block of memory. It is an object (like a table): you must create it, it can have its own metatable, and you can detect when it is being collected. A full userdata is only equal to itself (under raw equality).

When Lua collects a full userdata with a gc metamethod, Lua calls the metamethod and marks the userdata as finalized. When this userdata is collected again then Lua frees its corresponding memory.

lua_newthread

[-0, +1, m]

lua_State *lua_newthread (lua_State *L);

Creates a new thread, pushes it on the stack, and returns a pointer to a lua_State that represents this new thread. The new state returned by this function shares with the original state all global objects (such as tables), but has an independent execution stack.

There is no explicit function to close or to destroy a thread. Threads are subject to garbage collection, like any Lua object.

lua_pushthread

[-0, +1, -]

int lua_pushthread (lua_State *L);

Pushes the thread represented by L onto the stack. Returns 1 if this thread is the main thread of its state.

判断值的类型

lua_type

[-0, +0, -]

int lua_type (lua_State *L, int index);

Returns the type of the value in the given acceptable index, or LUA_TNONE for a non-valid index (that is, an index to an “empty” stack position). The types returned by lua_type are coded by the following constants defined in lua.h: LUA_TNIL, LUA_TNUMBER, LUA_TBOOLEAN, LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION, LUA_TUSERDATA, LUA_TTHREAD, and LUA_TLIGHTUSERDATA.

lua_typename

[-0, +0, -]

const char *lua_typename  (lua_State *L, int tp);

Returns the name of the type encoded by the value tp, which must be one the values returned by lua_type.

lua_isnil

[-0, +0, -]

int lua_isnil (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is nil, and 0 otherwise.

lua_isnoneornil

[-0, +0, -]

int lua_isnoneornil (lua_State *L, int index);

Returns 1 if the given acceptable index is not valid (that is, it refers to an element outside the current stack) or if the value at this index is nil, and 0 otherwise.

lua_isboolean

[-0, +0, -]

int lua_isboolean (lua_State *L, int index);

Returns 1 if the value at the given acceptable index has type boolean, and 0 otherwise.

lua_isnumber

[-0, +0, -]

int lua_isnumber (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a number or a string convertible to a number, and 0 otherwise.

lua_isstring

[-0, +0, -]

int lua_isstring (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a string or a number (which is always convertible to a string), and 0 otherwise.

lua_istable

[-0, +0, -]

int lua_istable (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a table, and 0 otherwise.

lua_iscfunction

[-0, +0, -]

int lua_iscfunction (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a C function, and 0 otherwise.

lua_isfunction

[-0, +0, -]

int lua_isfunction (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a function (either C or Lua), and 0 otherwise.

lua_islightuserdata

[-0, +0, -]

int lua_islightuserdata (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a light userdata, and 0 otherwise.

lua_isuserdata

[-0, +0, -]

int lua_isuserdata (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a userdata (either full or light), and 0 otherwise.

lua_isthread

[-0, +0, -]

int lua_isthread (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a thread, and 0 otherwise.

Lua 中的值转换为 C 类型

lua_toboolean

[-0, +0, -]

int lua_toboolean (lua_State *L, int index);

Converts the Lua value at the given acceptable index to a C boolean value (0 or 1). Like all tests in Lua, lua_toboolean returns 1 for any Lua value different from false and nil; otherwise it returns 0. It also returns 0 when called with a non-valid index. (If you want to accept only actual boolean values, use lua_isboolean to test the value’s type.)

lua_tointeger

[-0, +0, -]

lua_Integer lua_tointeger (lua_State *L, int index);

Converts the Lua value at the given acceptable index to the signed integral type lua_Integer. The Lua value must be a number or a string convertible to a number (see §2.2.1); otherwise, lua_tointeger returns 0.

If the number is not an integer, it is truncated in some non-specified way.

lua_tonumber

[-0, +0, -]

lua_Number lua_tonumber (lua_State *L, int index);

Converts the Lua value at the given acceptable index to the C type lua_Number (see lua_Number). The Lua value must be a number or a string convertible to a number (see §2.2.1); otherwise, lua_tonumber returns 0.

lua_tolstring

[-0, +0, m]

const char *lua_tolstring (lua_State *L, int index, size_t *len);

Converts the Lua value at the given acceptable index to a C string. If len is not NULL, it also sets *len with the string length. The Lua value must be a string or a number; otherwise, the function returns NULL. If the value is a number, then lua_tolstring also changes the actual value in the stack to a string. (This change confuses lua_next when lua_tolstring is applied to keys during a table traversal.)

lua_tolstring returns a fully aligned pointer to a string inside the Lua state. This string always has a zero (‘\0’) after its last character (as in C), but can contain other zeros in its body. Because Lua has garbage collection, there is no guarantee that the pointer returned by lua_tolstring will be valid after the corresponding value is removed from the stack.

lua_tostring

[-0, +0, m]

const char *lua_tostring (lua_State *L, int index);

Equivalent to lua_tolstring with len equal to NULL.

lua_tocfunction

[-0, +0, -]

lua_CFunction lua_tocfunction (lua_State *L, int index);

Converts a value at the given acceptable index to a C function. That value must be a C function; otherwise, returns NULL.

lua_touserdata

[-0, +0, -]

void *lua_touserdata (lua_State *L, int index);

If the value at the given acceptable index is a full userdata, returns its block address. If the value is a light userdata, returns its pointer. Otherwise, returns NULL.

lua_tothread

[-0, +0, -]

lua_State *lua_tothread (lua_State *L, int index);

Converts the value at the given acceptable index to a Lua thread (represented as lua_State*). This value must be a thread; otherwise, the function returns NULL.

lua_topointer

[-0, +0, -]

const void *lua_topointer (lua_State *L, int index);

Converts the value at the given acceptable index to a generic C pointer (void*). The value can be a userdata, a table, a thread, or a function; otherwise, lua_topointer returns NULL. Different objects will give different pointers. There is no way to convert the pointer back to its original value.

Typically this function is used only for debug information.

操作符

lua_equal

[-0, +0, e]

int lua_equal (lua_State *L, int index1, int index2);

Returns 1 if the two values in acceptable indices index1 and index2 are equal, following the semantics of the Lua == operator (that is, may call metamethods). Otherwise returns 0. Also returns 0 if any of the indices is non valid.

lua_rawequal

[-0, +0, -]

int lua_rawequal (lua_State *L, int index1, int index2);

Returns 1 if the two values in acceptable indices index1 and index2 are primitively equal (that is, without calling metamethods). Otherwise returns 0. Also returns 0 if any of the indices are non valid.

lua_lessthan

[-0, +0, e]

int lua_lessthan (lua_State *L, int index1, int index2);

Returns 1 if the value at acceptable index index1 is smaller than the value at acceptable index index2, following the semantics of the Lua < operator (that is, may call metamethods). Otherwise returns 0. Also returns 0 if any of the indices is non valid.

lua_concat

[-n, +1, e]

void lua_concat (lua_State *L, int n);

Concatenates the n values at the top of the stack, pops them, and leaves the result at the top. If n is 1, the result is the single value on the stack (that is, the function does nothing); if n is 0, the result is the empty string. Concatenation is performed following the usual semantics of Lua (see §2.5.4).

lua_objlen

[-0, +0, -]

size_t lua_objlen (lua_State *L, int index);

Returns the “length” of the value at the given acceptable index: for strings, this is the string length; for tables, this is the result of the length operator (‘#’); for userdata, this is the size of the block of memory allocated for the userdata; for other values, it is 0.

table 操作

lua_createtable

[-0, +1, m]

void lua_createtable (lua_State *L, int narr, int nrec);

Creates a new empty table and pushes it onto the stack. The new table has space pre-allocated for narr array elements and nrec non-array elements. This pre-allocation is useful when you know exactly how many elements the table will have. Otherwise you can use the function lua_newtable.

lua_newtable

[-0, +1, m]

void lua_newtable (lua_State *L);

Creates a new empty table and pushes it onto the stack. It is equivalent to lua_createtable(L, 0, 0).

lua_gettable

[-1, +1, e]

void lua_gettable (lua_State *L, int index);

Pushes onto the stack the value t[k], where t is the value at the given valid index and k is the value at the top of the stack.

This function pops the key from the stack (putting the resulting value in its place). As in Lua, this function may trigger a metamethod for the “index” event (see §2.8).

lua_getfield

[-0, +1, e]

void lua_getfield (lua_State *L, int index, const char *k);

Pushes onto the stack the value t[k], where t is the value at the given valid index. As in Lua, this function may trigger a metamethod for the “index” event (see §2.8).

lua_rawget

[-1, +1, -]

void lua_rawget (lua_State *L, int index);

Similar to lua_gettable, but does a raw access (i.e., without metamethods).

lua_rawgeti

[-0, +1, -]

void lua_rawgeti (lua_State *L, int index, int n);

Pushes onto the stack the value t[n], where t is the value at the given valid index. The access is raw; that is, it does not invoke metamethods.

lua_settable

[-2, +0, e]

void lua_settable (lua_State *L, int index);

Does the equivalent to t[k] = v, where t is the value at the given valid index, v is the value at the top of the stack, and k is the value just below the top.

This function pops both the key and the value from the stack. As in Lua, this function may trigger a metamethod for the “newindex” event (see §2.8).

lua_setfield

[-1, +0, e]

void lua_setfield (lua_State *L, int index, const char *k);

Does the equivalent to t[k] = v, where t is the value at the given valid index and v is the value at the top of the stack.

This function pops the value from the stack. As in Lua, this function may trigger a metamethod for the “newindex” event (see §2.8).

lua_rawset

[-2, +0, m]

void lua_rawset (lua_State *L, int index);

Similar to lua_settable, but does a raw assignment (i.e., without metamethods).

lua_rawseti

[-1, +0, m]

void lua_rawseti (lua_State *L, int index, int n);

Does the equivalent of t[n] = v, where t is the value at the given valid index and v is the value at the top of the stack.

This function pops the value from the stack. The assignment is raw; that is, it does not invoke metamethods.

lua_next

[-1, +(2|0), e]

int lua_next (lua_State *L, int index);

Pops a key from the stack, and pushes a key-value pair from the table at the given index (the “next” pair after the given key). If there are no more elements in the table, then lua_next returns 0 (and pushes nothing).

A typical traversal looks like this:

     /* table is in the stack at index 't' */
     lua_pushnil(L);  /* first key */
     while (lua_next(L, t) != 0) {
       /* uses 'key' (at index -2) and 'value' (at index -1) */
       printf("%s - %s\n",
              lua_typename(L, lua_type(L, -2)),
              lua_typename(L, lua_type(L, -1)));
       /* removes 'value'; keeps 'key' for next iteration */
       lua_pop(L, 1);
     }

While traversing a table, do not call lua_tolstring directly on a key, unless you know that the key is actually a string. Recall that lua_tolstring changes the value at the given index; this confuses the next call to lua_next.

元表

lua_getmetatable

[-0, +(0|1), -]

int lua_getmetatable (lua_State *L, int index);

Pushes onto the stack the metatable of the value at the given acceptable index. If the index is not valid, or if the value does not have a metatable, the function returns 0 and pushes nothing on the stack.

lua_setmetatable

[-1, +0, -]

int lua_setmetatable (lua_State *L, int index);

Pops a table from the stack and sets it as the new metatable for the value at the given acceptable index.

函数环境

lua_getfenv

[-0, +1, -]

void lua_getfenv (lua_State *L, int index);

Pushes onto the stack the environment table of the value at the given index.

lua_setfenv

[-1, +0, -]

int lua_setfenv (lua_State *L, int index);

Pops a table from the stack and sets it as the new environment for the value at the given index. If the value at the given index is neither a function nor a thread nor a userdata, lua_setfenv returns 0. Otherwise it returns 1.

函数调用

lua_call

[-(nargs + 1), +nresults, e]

void lua_call (lua_State *L, int nargs, int nresults);

Calls a function.

To call a function you must use the following protocol: first, the function to be called is pushed onto the stack; then, the arguments to the function are pushed in direct order; that is, the first argument is pushed first. Finally you call lua_call; nargs is the number of arguments that you pushed onto the stack. All arguments and the function value are popped from the stack when the function is called. The function results are pushed onto the stack when the function returns. The number of results is adjusted to nresults, unless nresults is LUA_MULTRET. In this case, all results from the function are pushed. Lua takes care that the returned values fit into the stack space. The function results are pushed onto the stack in direct order (the first result is pushed first), so that after the call the last result is on the top of the stack.

Any error inside the called function is propagated upwards (with a longjmp).

The following example shows how the host program can do the equivalent to this Lua code:

     a = f("how", t.x, 14)

Here it is in C:

     lua_getfield(L, LUA_GLOBALSINDEX, "f"); /* function to be called */
     lua_pushstring(L, "how");                        /* 1st argument */
     lua_getfield(L, LUA_GLOBALSINDEX, "t");   /* table to be indexed */
     lua_getfield(L, -1, "x");        /* push result of t.x (2nd arg) */
     lua_remove(L, -2);                  /* remove 't' from the stack */
     lua_pushinteger(L, 14);                          /* 3rd argument */
     lua_call(L, 3, 1);     /* call 'f' with 3 arguments and 1 result */
     lua_setfield(L, LUA_GLOBALSINDEX, "a");        /* set global 'a' */

Note that the code above is “balanced”: at its end, the stack is back to its original configuration. This is considered good programming practice.

lua_pcall

[-(nargs + 1), +(nresults|1), -]

int lua_pcall (lua_State *L, int nargs, int nresults, int errfunc);

Calls a function in protected mode.

Both nargs and nresults have the same meaning as in lua_call. If there are no errors during the call, lua_pcall behaves exactly like lua_call. However, if there is any error, lua_pcall catches it, pushes a single value on the stack (the error message), and returns an error code. Like lua_call, lua_pcall always removes the function and its arguments from the stack.

If errfunc is 0, then the error message returned on the stack is exactly the original error message. Otherwise, errfunc is the stack index of an error handler function. (In the current implementation, this index cannot be a pseudo-index.) In case of runtime errors, this function will be called with the error message and its return value will be the message returned on the stack by lua_pcall.

Typically, the error handler function is used to add more debug information to the error message, such as a stack traceback. Such information cannot be gathered after the return of lua_pcall, since by then the stack has unwound.

The lua_pcall function returns 0 in case of success or one of the following error codes (defined in lua.h):

• LUA_ERRRUN: a runtime error.
• LUA_ERRMEM: memory allocation error. For such errors, Lua does not call the error handler function.
• LUA_ERRERR: error while running the error handler function.

lua_cpcall

[-0, +(0|1), -]

int lua_cpcall (lua_State *L, lua_CFunction func, void *ud);

Calls the C function func in protected mode. func starts with only one element in its stack, a light userdata containing ud. In case of errors, lua_cpcall returns the same error codes as lua_pcall, plus the error object on the top of the stack; otherwise, it returns zero, and does not change the stack. All values returned by func are discarded.

chunk

lua_Reader

typedef const char * (*lua_Reader) (lua_State *L,
                                    void *data,
                                    size_t *size);

The reader function used by lua_load. Every time it needs another piece of the chunk, lua_load calls the reader, passing along its data parameter. The reader must return a pointer to a block of memory with a new piece of the chunk and set size to the block size. The block must exist until the reader function is called again. To signal the end of the chunk, the reader must return NULL or set size to zero. The reader function may return pieces of any size greater than zero.

lua_load

[-0, +1, -]

int lua_load (lua_State *L,
              lua_Reader reader,
              void *data,
              const char *chunkname);

Loads a Lua chunk. If there are no errors, lua_load pushes the compiled chunk as a Lua function on top of the stack. Otherwise, it pushes an error message. The return values of lua_load are:

• 0: no errors;
• LUA_ERRSYNTAX: syntax error during pre-compilation;
• LUA_ERRMEM: memory allocation error.

This function only loads a chunk; it does not run it.

lua_load automatically detects whether the chunk is text or binary, and loads it accordingly (see program luac).

The lua_load function uses a user-supplied reader function to read the chunk (see lua_Reader). The data argument is an opaque value passed to the reader function.

The chunkname argument gives a name to the chunk, which is used for error messages and in debug information (see §3.8).

lua_Writer

typedef int (*lua_Writer) (lua_State *L,
                           const void* p,
                           size_t sz,
                           void* ud);

The type of the writer function used by lua_dump. Every time it produces another piece of chunk, lua_dump calls the writer, passing along the buffer to be written (p), its size (sz), and the data parameter supplied to lua_dump.

The writer returns an error code: 0 means no errors; any other value means an error and stops lua_dump from calling the writer again.

lua_dump

[-0, +0, m]

int lua_dump (lua_State *L, lua_Writer writer, void *data);

Dumps a function as a binary chunk. Receives a Lua function on the top of the stack and produces a binary chunk that, if loaded again, results in a function equivalent to the one dumped. As it produces parts of the chunk, lua_dump calls function writer (see lua_Writer) with the given data to write them.

The value returned is the error code returned by the last call to the writer; 0 means no errors.

This function does not pop the Lua function from the stack.

协程

lua_newthread

[-0, +1, m]

lua_State *lua_newthread (lua_State *L);

Creates a new thread, pushes it on the stack, and returns a pointer to a lua_State that represents this new thread. The new state returned by this function shares with the original state all global objects (such as tables), but has an independent execution stack.

There is no explicit function to close or to destroy a thread. Threads are subject to garbage collection, like any Lua object.

lua_resume

[-?, +?, -]

int lua_resume (lua_State *L, int narg);

Starts and resumes a coroutine in a given thread.

To start a coroutine, you first create a new thread (see lua_newthread); then you push onto its stack the main function plus any arguments; then you call lua_resume, with narg being the number of arguments. This call returns when the coroutine suspends or finishes its execution. When it returns, the stack contains all values passed to lua_yield, or all values returned by the body function. lua_resume returns LUA_YIELD if the coroutine yields, 0 if the coroutine finishes its execution without errors, or an error code in case of errors (see lua_pcall). In case of errors, the stack is not unwound, so you can use the debug API over it. The error message is on the top of the stack. To restart a coroutine, you put on its stack only the values to be passed as results from yield, and then call lua_resume.

lua_yield

[-?, +?, -]

int lua_yield  (lua_State *L, int nresults);

Yields a coroutine.

This function should only be called as the return expression of a C function, as follows:

     return lua_yield (L, nresults);

When a C function calls lua_yield in that way, the running coroutine suspends its execution, and the call to lua_resume that started this coroutine returns. The parameter nresults is the number of values from the stack that are passed as results to lua_resume.

lua_status

[-0, +0, -]

int lua_status (lua_State *L);

Returns the status of the thread L.

The status can be 0 for a normal thread, an error code if the thread finished its execution with an error, or LUA_YIELD if the thread is suspended.

Lua 全局变量

lua_register

[-0, +0, e]

void lua_register (lua_State *L,
                   const char *name,
                   lua_CFunction f);

Sets the C function f as the new value of global name. It is defined as a macro:

     #define lua_register(L,n,f) \
            (lua_pushcfunction(L, f), lua_setglobal(L, n))

lua_setglobal

[-1, +0, e]

void lua_setglobal (lua_State *L, const char *name);

Pops a value from the stack and sets it as the new value of global name. It is defined as a macro:

     #define lua_setglobal(L,s)   lua_setfield(L, LUA_GLOBALSINDEX, s)

lua_getglobal

[-0, +1, e]

void lua_getglobal (lua_State *L, const char *name);

Pushes onto the stack the value of the global name. It is defined as a macro:

     #define lua_getglobal(L,s)  lua_getfield(L, LUA_GLOBALSINDEX, s)

其他

lua_gc

[-0, +0, e]

int lua_gc (lua_State *L, int what, int data);

Controls the garbage collector.

This function performs several tasks, according to the value of the parameter what:

• LUA_GCSTOP: stops the garbage collector.
• LUA_GCRESTART: restarts the garbage collector.
• LUA_GCCOLLECT: performs a full garbage-collection cycle.
• LUA_GCCOUNT: returns the current amount of memory (in Kbytes) in use by Lua.
• LUA_GCCOUNTB: returns the remainder of dividing the current amount of bytes of memory in use by Lua by 1024.
• LUA_GCSTEP: performs an incremental step of garbage collection. The step “size” is controlled by data (larger values mean more steps) in a non-specified way. If you want to control the step size you must experimentally tune the value of data. The function returns 1 if the step finished a garbage-collection cycle.
• LUA_GCSETPAUSE: sets data as the new value for the pause of the collector (see §2.10). The function returns the previous value of the pause.
• LUA_GCSETSTEPMUL: sets data as the new value for the step multiplier of the collector (see §2.10). The function returns the previous value of the step multiplier.

lua_error

[-1, +0, v]

int lua_error (lua_State *L);

Generates a Lua error. The error message (which can actually be a Lua value of any type) must be on the stack top. This function does a long jump, and therefore never returns. (see luaL_error).