In GT.M, calls to C language routines may be made with the following syntax:

DO &[packagename.]name[^name][parameter-list]

or as an expression element,

$&[packagename.]name[^name][parameter-list]

Where packagename, like the name elements is a valid M name. Because of the parsing conventions of M, the identifier between the ampersand (&) and the optional parameter-list has precisely constrained punctuation - a later section describes how to transform this into a more richly punctuated name should that be appropriate for the called function. While the intent of the syntax is to permit the name^name to match an M labelref, there is no semantic implication to any use of the up-arrow (^). For more information on M names, labelrefs and parameter-lists, refer toChapter 5: “General Language Features of M”.

Example:

;Call external routine rtn1
DO &rtn1
;Call int^exp in package "mathpak" with one parameter: the expression val/2
DO &mathpak.int^exp(val/2)
;Call the routine sqrt with the value "2"
WRITE $&sqrt(2)
;Call the routine get parms, with the parameter "INPUT" and the variable "inval", passed by reference.
DO &getparms("INPUT",.inval)
;Call program increment in package "mathpak" without specifying a value for the first argument and the variable "outval" passed by reference as the second argument. All arguments which do not specify a value translate to default values in the increment program.  
Do &mathpak.increment(,.outval) 

The called routines follow the C calling conventions. They must be compiled as position independent code and linked as a shareable library.