Syntax :

=CALL(dll_name,function_name,type_string,arg1,...,argN)

Where 
dll_name  is the name of the DLL that contains the function. This name must contain a full path if the DLL file is not located in the Windows folder, the System folder or a folder named in the PATH environment string.

function_name is  name of the function.

Example:

=CALL("C:\Temp\MyDll","MyFunc","BJJB",25,3,98.65)

The Excel4 function, contained in the file  ...\Office\Xlcall32.dll, is a callback function which exposes all XLM functions and macros to stand-alone add-in DLLs (XLLs).

C prototype :

int cdecl Excel4(int xlfn, LPXLOPER operRes, int count,...);

Where
xlfn is a number indicating which Excel function or macro to call.

operRes is pointer to an XLOPER structure which receives the result of the function / macro

count is the number of arguments

The arguments following count must be pointers to XLOPER structures ("LPXLOPER").

Excel4 returns a number which indicates wether the call has successed or not. We won't use this value in the following formulas.

The main XLM information functions which can be used in this way are the following:

Using XLM functions in worksheets avoids having to write VBA (or even XLM) user defined functions for getting any information about a cell, a sheet or Excel's environment. XLM functions invoked directly with CALL and Excel4 are faster than custom VBA functions.

Here are some examples:

Warning : don't build the following formulas in the Function Wizard; instead, type them directly in the cells or in the formula bar.

=CALL("Xlcall32","Excel4","2JRJRR#",185,,2,18,A1)
returns the name of the font used in cell A1 (calls the GET.CELL function)

=CALL("Xlcall32","Excel4","2JRJRR#",185,,2,24,A1)
returns the color index of the first character in A1 (GET.CELL)

=CALL("Xlcall32","Excel4","2JRJRR#",268,,2,4,
CALL("Xlcall32","Excel4","2JRJRR#",185,,2,66,INDIRECT("A1")))
returns the number of sheets in the workbook (GET.WORKBOOK and GET.CELL)

=CALL("Xlcall32","Excel4","2JRJR#",188,,1,50)
returns the total number of pages that would be printed based on current settings (GET.DOCUMENT)

The first two  formulas above are not immediately recalculated (because changing a font or a color in a cell doesn't cause any calculation at all). Formulas 3 and 4 are recalculated when the value in A1 changes. The 5th formula recalculates only when you delete a sheet in the workbook (inserting a sheet doesn't cause a recalculation).

This formula returns a "static" (not volatile) random value in the range [0,1).  It calls the standard worksheet function RAND(), but as an XLM function. If you want to update the returned random value(s), just press Ctrl-Alt-F9.

=CALL("Xlcall32","Excel4","2JRJ",63)

The following returns a random integer in the range [1,100] (still static):

=INT(CALL("Xlcall32","Excel4","2JRJ",63)*100)+1

DLL functions called with a number sign (#) in the type string can deal with the contents of the cell in which they are entered, whereas normal worksheet formulas and VBA functions can't do it without using Circular References:

=CALL("Xlcall32","Excel4","2JRJRR#",185,,2,5,INDIRECT("RC",0))

(equivalent to the XLM formula =GET.CELL(5,INDIRECT("A1"))

The "RC" refers, in R1C1 reference style, to the cell which the formula is entered.

This feature allows to use formulae like the following, which "stores" the highest value entered in the cell A1. You don't need to enable circular references in the Options menu.

=MAX(A1,CALL("Xlcall32","Excel4","2JRJRR#",185,,2,5,INDIRECT("RC",0)))

The GET.CELL XLM function, which retrieves several informations about cells (font, size, etc.), can be used in array formulas.  For instance, you can use it to count the number of cells in a range whith text colored in red.

The syntax of these array formulae is relatively "tortured", because GET.CELL is supposed to work only with single cells, not ranges [... I just say  is supposed to be use].

The following formulae are based on GET.CELL(24,CellRef), which returns the color index of the first character in CellRef (index of red = 3). In our particular syntax, this function corresponds to:

=CALL("Xlcall32","Excel4","2JRJRR#",185,,2,24,CellRef)

{=SUM(N(CALL("Xlcall32","Excel4","2JRJRR#",185,,2,24,
TRANSPOSE(OFFSET(Rg,ROW(INDIRECT("1:"&ROWS(Rg)))-1,,1)))=3))}

{=SUM(N(CALL("Xlcall32","Excel4","2JRJRR#",185,,2,24,
TRANSPOSE(OFFSET(Rg,,COLUMN(Rg)-COLUMN(INDEX(Rg,,1)),,1)))=3))}

{=SUM(N(CALL("Xlcall32","Excel4","2JRJRR#",185,,2,24,
TRANSPOSE(OFFSET(Rg,ROW(INDIRECT("1:"&ROWS(Rg)))-1,
COLUMN(Rg)-COLUMN(INDEX(Rg,,1)),1,1)))=3))}

A shorter, though slightly slower (but still much faster than a custom VBA function), version of this formula follows:

{=SUM(N(CALL("Xlcall32", "Excel4", "2JRJRR#", 185,,2,24,TRANSPOSE(INDIRECT(ADDRESS(ROW(Rg),COLUMN(Rg),4))))=3))}

When you change the color of a cell, the result of the formula is not updated automatically. You must wait for the next calculation in the sheet, or simply press F9.