The easiest way to call into a C library is to define bindings using a pair of parsing words:

LIBRARY:

FUNCTION:

FUNCTION-ALIAS:

The above parsing words create word definitions which call a lower-level word; you can use it directly, too:

alien-invoke ( args... return library function parameters varargs? -- return... )

Sometimes it is necessary to invoke a C function pointer, rather than a named C function:

alien-indirect ( args... funcptr return parameters abi -- return... )

There are some details concerning the conversion of Factor objects to C values, and vice versa. See Passing data between Factor and C.