format_spec

    Characters to be written literally to the output or converted as
    specified in the . . . arguments.

  . . .

    Optional expressions whose resultant types correspond to
    conversion specifications given in the format specification.

    If no conversion specifications are given, you may omit the
    output sources. Otherwise, the function call must have exactly
    as many output sources as there are conversion specifications,
    and the conversion specifications must match the types of the
    output sources.

    Conversion specifications are matched to output sources in left-
    to-right order. Excess output pointers, if any, are ignored.