GT.M allows references to a GT.M database from programs written in other programming languages that run under UNIX.

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 caret (^).