types

Here we list all types from the C API in alphabetical order.

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_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_Debug

typedef struct lua_Debug {
  int event;
  const char *name;           /* (n) */
  const char *namewhat;       /* (n) */
  const char *what;           /* (S) */
  const char *source;         /* (S) */
  int currentline;            /* (l) */
  int nups;                   /* (u) number of upvalues */
  int linedefined;            /* (S) */
  int lastlinedefined;        /* (S) */
  char short_src[LUA_IDSIZE]; /* (S) */
  /* private part */
  <other fields>
} lua_Debug;

A structure used to carry different pieces of information about an active function. lua_getstack fills only the private part of this structure, for later use. To fill the other fields of lua_Debug with useful information, call lua_getinfo.

The fields of lua_Debug have the following meaning:

• source: If the function was defined in a string, then source is that string. If the function was defined in a file, then source starts with a '@' followed by the file name.

• short_src: a "printable" version of source, to be used in error messages.

• linedefined: the line number where the definition of the function starts.

• lastlinedefined: the line number where the definition of the function ends.

• what: the string "Lua" if the function is a Lua function, "C" if it is a C function, "main" if it is the main part of a chunk, and "tail" if it was a function that did a tail call. In the latter case, Lua has no other information about the function.

• currentline: the current line where the given function is executing. When no line information is available, currentline is set to -1.

• name: a reasonable name for the given function. Because functions in Lua are first-class values, they do not have a fixed name: some functions can be the value of multiple global variables, while others can be stored only in a table field. The lua_getinfo function checks how the function was called to find a suitable name. If it cannot find a name, then name is set to NULL.

• namewhat: explains the name field. The value of namewhat can be "global", "local", "method", "field", "upvalue", or "" (the empty string), according to how the function was called. (Lua uses the empty string when no other option seems to apply.)

• nups: the number of upvalues of the function.

lua_Hook

typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);

Type for debugging hook functions.

Whenever a hook is called, its ar argument has its field event set to the specific event that triggered the hook. Lua identifies these events with the following constants: LUA_HOOKCALL, LUA_HOOKRET, LUA_HOOKTAILRET, LUA_HOOKLINE, and LUA_HOOKCOUNT. Moreover, for line events, the field currentline is also set. To get the value of any other field in ar, the hook must call lua_getinfo. For return events, event can be LUA_HOOKRET, the normal value, or LUA_HOOKTAILRET. In the latter case, Lua is simulating a return from a function that did a tail call; in this case, it is useless to call lua_getinfo.

While Lua is running a hook, it disables other calls to hooks. Therefore, if a hook calls back Lua to execute a function or a chunk, this execution occurs without any calls to hooks.

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_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_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_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_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.