EXTERN is similar to the MASM directive EXTRN and the C keyword extern: it is used to declare a symbol which is not defined anywhere in the module being assembled, but is assumed to be defined in some other module and needs to be referred to by this one. Not every object-file format can support external variables: the bin format cannot.

The EXTERN directive takes as many arguments as you like. Each argument is the name of a symbol:

        extern _printf
        extern _sscanf, _fscanf

Some object-file formats provide extra features to the EXTERN directive. In all cases, the extra features are used by suffixing a colon to the symbol name followed by object-format specific text. For example, the obj format allows you to declare that the default segment base of an external should be the group dgroup by means of the directive

        extern _variable:wrt dgroup

The primitive form of EXTERN differs from the user-level form only in that it can take only one argument at a time: the support for multiple arguments is implemented at the preprocessor level.

You can declare the same variable as EXTERN more than once: NASM will quietly ignore the second and later redeclarations. You can’t declare a variable as EXTERN as well as something else, though.