Lua Generic Call

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Using standard Lua API, calling a chunk function while passing input parameters and retrieving output results requires a lot of code and complete knowledge about Lua stack. As an example, here is how to ask Lua to perform the multiplication of 3 by 2.5, checking for possible syntax and runtime errors:
const char* errmsg = NULL;
double result;
lua_settop(L, 0);
if(luaL_loadstring(L, "local a,b = ...; return a*b"))
  errmsg = lua_tostring(L, -1);
  lua_pushinteger(L, 3);
  lua_pushnumber(L, 2.5);
  if(lua_pcall(L, 2, 1, 0))
    errmsg = lua_tostring(L, -1);
    result = lua_tonumber(L, -1);
The generic call feature is primarily aimed to simplify such calling tasks. It allows replacing all the previous code with a single function call:
double result;
const char* errmsg = lua_genpcall(L, "local a,b = ...; return a*b",
  "%d %f > %lf", 3, 2.5, &result);
The second goal is to minimize, ideally to only one, the number of functions needed in the API. This can for example simplify dynamic loading of Lua shared library, because for each exported function, you need to define a new type for the prototype of the function, instantiate a variable of this type and make a call to GetProcAddress or dlsym. However, for this job, it may be preferable to follow EasyManualLibraryLoad guideline.

The source code can be found here []


Four versions of the general call function are in fact defined, but normally you only use one of them: As with all include files under Windows OS, a compilation switch automatically redirects lua_gencall to either lua_gencallA or lua_gencallW depending whether UNICODE is defined or not. It is also possible to compile the library file without wide character support.

General form

Each of these functions has a variable number of arguments, and three fixed ones.
1. lua_State* L: a pointer to an instance of a Lua state. If the pointer is NULL, the function automatically calls lua_newstate to create a new instance, and will also call lua_close to release its memory, unless the pointer to the instance is retrieved with %S option (see after). If the state is freed and an error occurred before, the function allocates a buffer to copy the error message, which must be closed with free.
2. script string: it contains some piece of Lua code to execute. It typically begins with parameter retrieval: local var1, var2, var3 = ...; and ends with returning results: return res1, res2, res2. If the pointer is NULL, it is equivalent to the empty script "".
3. format string: a string similar to the printf or scanf format strings, using the % character to describe the variable types of input and output values. If the pointer is NULL, it is equivalent to the empty format "".
4. zero or more value parameters. Input parameters are passed by value, while output results must be retrieved by passing addresses of variables. Allocation options may also change the expected types of variables.

For performance reasons, there is a cache of already compiled chunks. It is implemented as a Lua table in the Lua registry, indexed by chunk strings. So if you call several times lua_genpcall with the same script string, it is compiled only the first time. All successive calls will reuse the cached version. The cache table is not a weak table, to avoid having to recompile several times the same script, because of garbage collection going on. A drawback if that the cache may grow indefinitely when the script chunks can change arbitrary at runtime. This can for example happen on a server interpreter executing Lua chunks coming from a client program. But you can explicitly clear the cache by specifying it on the format string.

As with printf and even more with scanf, you must be very careful with the types of the arguments and the corresponding format specifications. Any mismatch can lead to unexpected results, or even worse, to an application crash.

The general form of the format string is this one:

"[ Directives < ] Inputs [ > Outputs ]"
In this general form, Directives, Inputs and Outputs are strings similar to printf and scanf formats, consisting in format items beginning with a percent sign. Directives and Outputs are optional parts. If they are present, there must be separated from Inputs with a < or a > character. Inputs can also be an empty string.

Input and output format strings

As with standard printf format specifications, each input or output format item consists of up to 6 fields, as in the next example. Directive items have fewer options and are explained later. Any blank character (space, tabulation, carriage return and line feed) is ignored and may be used to increase clarity in the format string. Other characters are either interpreted as explained in this chapter, or throw a Lua error when invalid.
1. A mandatory percent sign ('%')
2. An optional flags argument ('#')
3. An optional width argument ('12')
4. An optional precision argument ('.4')
5. An optional size modifier ('L')
6. A mandatory conversion character ('s')

The flag argument may be one of these characters:

The width parameter is used with strings, string lists and arrays. It represents the number of elements or characters of the memory buffer. It can be one of the following forms: the following forms:

The precision argument is used with numerical types to indicate the size in bytes of the C type. It is important for numerical arrays, and for output values. This is because in both cases, a pointer to a variable and not the value itself it passed. It has one of these forms:

The size modifier argument is an alternative to the numerical precision value, and changes the expected C type of the value. It can be one of these characters. Please refer to size table below for correspondences:

Most importantly, the conversion character specifies the type of data. It must be one of the following characters:

For numerical values, here are the default and modified underlying C types listed in the following table:

         (default)               'hh'       'h'       'l'           'L'
'f'      float                   ---        float     double        long double (*)
'd','i'  int                     char       short     long          int64_t (*)
'u'      unsigned                unsigned   unsigned  unsigned      uint64_t (*)
         int                     char       short     long         
'b'      C89: int                ---        char      int           ---  
         C99: _Bool                                                
         C++: bool                                                 
's','z'  gencallA: char*                                           
         gencallW: wchar_t* (*)  ---        char*     wchar_t* (*)  ---
(*) If supported by your compiler and enabled at compilation time

Other object values have the following associated C types:

Finally, for each parameter its expected type depends on whether it is on input or output direction, and on its width and flag arguments. Let be TYPE the basic C type as stated in previous 2 tables. Except for 'n' and 'k' conversion characters, the composed types are:
Width argument    (none)            number, '*' or '&'   number, '*' or '&'
Flag              (don't care)      (none)               '#' or '+'

Input             TYPE              const TYPE*          const TYPE*
Output            TYPE*             TYPE*                TYPE**

Directive format string

In the directive part of the format string, the following conversion characters (all uppercases) are supported:

Source code


Lua Generic call library has been placed under the same MIT license as Lua itself. This means that although the library is copyrighted by Olivetti Engineering SA, it is free software and can be used for both academic and commercial purposes at absolutely no cost.

Source files

The library distribution consists in just one C implementation file lgencall.c and one header file lgencall.h. There is also a testing file testwin.cpp, which includes all test examples of the next chapter, including Windows header file tchar.h. Using this utility header, it is possible to write code that compile for both ANSI and Unicode platforms.

The main C file includes ANSI standard files, and the public Lua API header files. Like other standard Lua libraries, no private feature is used, and the file can be compiled in both C and C++ languages. However, it requires the new C99 include file stdint.h to define fixed size integers. If your compiler does not support this, there are several free versions available on the WWW. [1] [2]

The source file can either be compiled together with the application, or placed inside Lua shared library if you can afford to recompile it.

Compilation switches

In header file lgencall.h are defined 3 compilation macros which are used to customize the library for your platform. Each parameter can either be changed in the file itself, or specified on the compiler's command line. A small explanation for it is present in the header file, listing the possible values. Also, the compilation is affected by the following standard macros: __cplusplus, INT_MAX, UINT_MAX and __STDC_VERSION__.


Directive elements

For sure, examples will help you to understand the different features. The first examples show the usage of various directive formats. Let us begin with the simplest one, a "Hello World" program of course:
lua_genpcall(NULL, "print 'Hello World!'", "%O<");
Here no Lua state is passed to the function, so it is automatically allocated. The %O in the directive part instructs to open standard libraries, with include the global print function. Since no %S option is passed, Lua state is freed at the end of the call. We haven't tested the return value, which is the error message.

Here is the same example, but as a more complete and realistic implementation:

lua_State* L = lua_open();
const char* errmsg = lua_genpcall(L, "print 'Hello World!'", "");
  fprintf(stderr, "Lua error: %s\n", errmsg);
Here, Lua state is manually created, filled with standard libraries and freed. The return error message is tested and printed in case of trouble.

Again the same example, but using only lua_genpcall:

lua_State* L;
lua_Alloc falloc;
lua_genpcall(NULL, NULL, "%O %S %&M<", &L, &falloc);
char* errmsg = lua_genpcall(L, "print 'Hello World!'", "%C<");
  fprintf(stderr, "Lua error: %s\n", errmsg);
  falloc(NULL, errmsg, 10, 0);
The first call will allocate a new Lua state, open standard libraries (%O), return the Lua state (%S) and also the memory allocation function (%&M). The second call prints the message and destroys Lua state (%C). Because of this, the error message (if present) is allocated with Lua allocation function, and not taken from the stack. Therefore, it is best to free it with the same function (passing 0 as the new size).

Input elements

There will be 6 examples of how to input data from C to Lua:
1. Numbers
2. Boolean, nil, simple strings and light userdata
3. C functions and call-backs
4. Numerical arrays
5. Advanced strings
6. String lists
For all these examples, we will suppose that Lua state is already open and that it will be closed at the end. We are not testing for return errors to simplify the coding.

1. Numbers

lua_genpcall(L, "for k,v in pairs{...} do print(k, type(v), v) end", 
  "%i %d %u %f %f", -4, 0xFFFFFFFF, 0xFFFFFFFF, 
  3.1415926535f, 3.1415926535);
1       number  -4
2       number  -1
3       number  4294967295
4       number  3.1415927410126
5       number  3.1415926535
The script chunk loops over the arguments and for each one prints its type and value, as long as the index. Here, five numerical arguments are passed: three integers, a floating point and a double floating point (all seen as type number by Lua). Because in Lua all numbers are stored as double, there is a truncation error in the value of Pi for the float argument. Please note the difference in behaviour between %d and %u for the value 0xFFFFFFFF. For the double parameter, it is not necessary to specify %lf instead of %f here, because floating point numbers are always converted to double when passed to a variable argument function in C. Integers smaller than int are also automatically converted to int.

2. Boolean, nil, simple strings and light userdata

lua_genpcall(L, "for k,v in pairs{...} do print(k, type(v), v) end", 
  "%b %b %n %s %p", 0, 1, "Hello", L);
1       boolean false
2       boolean true
4       string  Hello
5       userdata        userdata: 0096CE70
Boolean values can be 0 or 1, or true and false if compiled in C++ or in C99 languages. The nil parameter %n is only present in the format string, no parameter is associated (it is not printed because function pairs skips nil values). The string is here supposed to be zero terminated, and the last parameter L (the Lua state) is just an example of a generic pointer.

3. C functions and call-backs

int cFunction(lua_State* L)
  printf("%s\n", luaL_checkstring(L, 1));
  return 1;
void pushMessage(lua_State* L, const void* ptr)
  lua_pushstring(L, *(const char**)ptr);
lua_genpcall(L, "local fct, msg = ...; fct(msg)", 
  "%c %k", cFunction, pushMessage, "Hello from C!");
The first Lua argument is of type function and is passed as a pointer to cFunction. The second argument is a call-back parameter consisting of the user function pushMessage and a string. Please note that the call-back function receives a pointer to the argument and not the argument itself!

4. Numerical arrays

short array[] = { 1,2,3 };
  "for k,v in pairs{...} do print(k, #v, table.concat(v, ', ')) end", 
  "%2hd %5.1u %*.*d", array, "Hello", 
  sizeof(array)/sizeof(array[0]), sizeof(array[0]), array);
1       2       1, 2
2       5       72, 101, 108, 108, 111
3       3       1, 2, 3
The code chunk prints the parameter index, the length of the array and a list with its values. For array data, it is necessary to specify the precision or a size modifier (unless it is the default type) and a width value. The first parameter declares a width of 2, therefore only the first two numbers of the short array are received. The second parameter is a string (char[]), so its precision is 1 and the width is the string length. On the third argument, both width and precision are passed though the argument list since the format specifies '*'.

5. Advanced strings

unsigned char data[] = { 200, 100, 0, 3, 5, 0 };
lua_genpcall(L, "for k,v in pairs{...} do print(k, v:gsub('.', "
  "function(c) return '\\\\'.. c:byte() end)) end", 
  "%s %6s %*s %ls", "Hello", "P1\0P2", sizeof(data), data, L"été"); 
1       \72\101\108\108\111     5
2       \80\49\0\80\50\0        6
3       \200\100\0\3\5\0        6
4       \195\169\116\195\169    5 
Strings are not necessarily zero terminated arrays of char. Here the script chunk prints the argument index, then the string in which each byte is replaced by a backslash and its decimal value. Note that the backslash has to be escaped twice: first for C (the chunk passed to Lua is ... return '\\' ...), and second for Lua. The first argument is a zero terminated string; the second is a string containing a binary 0, specified by its length. The third argument is a binary array of data, for which the length is passed by argument. And the last argument is a wide character string, which is converted to a UTF-8 string or another form of multi-byte string, depending on how the module was compiled.

6. String lists

  "for k,v in pairs{...} do print(k, #v, table.concat(v, ',')) end",
  "%z  %7z %hz %*lz", "s1\0s2\0s3\0", "s4\0\0s5\0", 
  "c1\0c2\0c3\0", 7, L"w1\0\0w2\0"	);
1       3       s1,s2,s3
2       3       s4,,s5
3       3       c1,c2,c3
4       3       w1,,w2
An array of strings is expected to be a zero terminated list of zero terminated strings on the C side. In other words, it is a string containing one a more additional null characters inside it, delimiting elements. Because of this, it is impossible to support strings with embedded zeros. In the first example, no width is specified, so the string list automatically ends on the first double zero bytes. The second list has its second string element of length 0. In this case, if no width was provided in the format string, the array would be erroneously of length 1, because there are two consecutive zero bytes in the middle. By specifying the width to be 7 (so not counting the last zero byte, as in usual strings), the number of received array elements is correctly 3. The third example simply specifies that the string list is of type char*. Otherwise, on Unicode support, lua_genpcallW expects wide character strings. The last list is a wide character version specifying its length with an addition '*' parameter.

Output elements

In output mode, the main differences are that we must normally pass pointer to variables and not values, and we should always specify the precision field. Beware: on little endian processors like Intel ones, passing a wrong precision value might work anyway; but it will surely fail on big endian platforms! There are again 6 examples, demonstrating the same data types as for input elements.

1. Numbers

char var1; unsigned short var2; int var3;
float var4; double var5;
lua_genpcall(L, "return 1, 2, 3, 4, 5", ">%hhd %hu %d %f %lf", 
  &var1, &var2, &var3, &var4, &var5);
printf("%d %u %d %f %f\n", var1, var2, var3, var4, var5);
1 2 3 4.000000 5.000000
This sample retrieves 5 numerical values of different types and sizes. The second variable is unsigned, var1 and var3 are signed integers, and the last 2 are floating point numbers. In this case, the %lf format is mandatory!

2. Boolean, nil, simple strings and light userdata

bool bool1; int bool2; 
const char* str; void* ptr;
lua_genpcall(L, "return true, false, 'dummy', 'Hello', io.stdin", 
  ">%hb %lb %n %+s %p", &bool1, &bool2, &str, &ptr);
printf("%d %d %s %p\n", bool1, bool2, str, ptr);
1 0 Hello 00975598
In this example the first two parameters retrieved are two Boolean values, of C different types. The third return value is discarded because of the %n format. A 'Hello' string is received through Lua stack with %+s idiom. And the last result value, a userdatum value, is got by address into a generic pointer.

3. C functions and call-backs

void getMessage(lua_State* L, int idx, void* ptr)
  *(const char**)ptr = lua_tostring(L, idx);
lua_CFunction fct;
const char* msg;
lua_genpcall(L, "return print, 'Hello World!'", 
  ">%c %k", &fct, getMessage, &msg);
lua_pushstring(L, msg);
This sample is a complicated way to implement the Hello World program. The first return value is a pointer to a Lua registered C function, the global print function. The second value, a simple string, is retrieved through a callback function, which receives as its ptr argument the address of variable msg. Then we can print the message by pushing the message onto Lua stack and call the print function directly by C (which is certainly not a good practice in normal situations).

4. Numerical arrays

unsigned int int_a[3];
bool bool_a[4];
char* str; 
short* pshort;
int short_len;
int bool_len = sizeof(bool_a)/sizeof(bool_a[0]);
lua_genpcall(L, "return {1,2,3,4},{72,101,108,108,111,0}, {5,6,7}, {false,true}", 
  ">%3u %+.1d %#&hd %&.*b", &int_a, &str, &short_len, &pshort, 
  &bool_len, sizeof(bool_a[0]), &bool_a);
printf("int_a = {%u,%u,%u}\nstr = %s\npshort[%d]=%d\nbool_a = #%d:{%d,%d,%d,%d}\n", 
  int_a[0], int_a[1], int_a[2], str, short_len-1, pshort[short_len-1],
  bool_len, bool_a[0], bool_a[1], bool_a[2], bool_a[3]);
int_a = {1,2,3}
str = Hello
bool_a = #2:{0,1,204,204}
The first array is allocated in the C stack and filled up by Lua up to 3 values (the last one is lost). The second is allocated on Lua stack and because it is of type char, its precision is set to 1. Note that because the '+' sign is present, it is not necessary to specify a width. In the third array, the real length is returned (because of '&') in addition to a pointer allocated by the current Lua allocation function (instructed by '#'). We have to free this buffer after use. In the fourth Boolean array, the precision is passed by the '*' feature. More interesting, the width argument is passed in both input and output directions with the '&' sign. The value of bool_len must be initialized to the size of the array before the call, since we are using a C stack allocated buffer. Because the buffer is larger than the returned array, its last two values will remain not initialized.

5. Advanced strings

const char *str1;
char *str2;
char str3[10];
unsigned char data[6];
int len = sizeof(data);
wchar_t* wstr;
lua_genpcall(L, "return 'Hello', ' Wor', 'ld!', '\\0\\5\\200\\0', 'Unicode'",
  ">%+s %#s %*s %&s %+ls", &str1, &str2, sizeof(str3), str3, &len, data, &wstr);
printf("%s%s%s\ndata (%d bytes): %02X %02X %02X %02X %02X\nwstr = %S\n", 
  str1, str2, str3, len, data[0],data[1],data[2],data[3],data[4], wstr);
Hello World!
data (4 bytes): 00 05 C8 00 00
wstr = Unicode
This sample retrieves five strings in different ways. The first string is taken from Lua stack ('+' sign). The second is allocated by Lua current allocation function, and must therefore be freed after its use. The third one is taken from the C stack and the buffer size is passed through the '*' width specification. The next return value is considered as a string on Lua side, but is defined as a raw byte buffer in C. Through the '&' mechanism, we both set the buffer size by initializing variable len, and get back the real data size after the call. Note that there is always an additional zero byte copied to the destination buffer (if there is enough place). The last value is a wide character string, placed on Lua stack.

6. String lists

void print_string_list(const char* title, const void* data, int fchar){
  printf("%-4s = {", title);
  if(fchar) {
    const char* str = (const char*)data;
      printf("'%s', ", str);
      str += strlen(str) + 1;
  else {
    const wchar_t* str = (const wchar_t*)data;
    while(*str) {
      printf("'%S', ", str);
      str += wcslen(str) + 1;
const char *str1;
char *str2;
char str3[10];
int len;
wchar_t* wstr;
lua_genpcall(L, "return {1,2,3},{4,5,6},{10,9,8,7},{11,12}",
  ">%+hz %+&z %*z %#lz", &str1, &len, &str2, 
  sizeof(str3)/sizeof(str3[0]), &str3, &wstr );
print_string_list("str1", str1, 1);
print_string_list("str2", str2, 1);
printf("len = %d\n", len);
print_string_list("str3", str3, 1);
print_string_list("wstr", wstr, 0);
str1 = {'1', '2', '3', }
str2 = {'4', '5', '6', }
len = 6
str3 = {'10', '9', '8', '7', }
wstr = {'11', '12', }
In this last example, the helper function print_string_list is just here to display the retrieved string lists in a readable form. The first string list is of type char*, whether or not the Unicode version is used, which is not the case for str2 and str3. The first list is allocated on Lua stack; the second one also, with in addition the retrieval of the string list length (counted without the last zero byte). The buffer of str3 is on the C stack; so its size is passed as an additional parameter instructed by the '*' flag. The last string list is a wide character version; its buffer is allocated with Lua allocator, requiring a call to free after use. You have certainly noted the strong similarities between string and string list parameters.

-- PatrickRapin

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Last edited January 10, 2010 3:04 pm GMT (diff)