format_spec

    Pointer to a string containing the format specification. The
    format specification consists of characters to be taken literally
    from the input or converted and placed in memory at the specified
    input sources.

  . . .

    Optional expressions that are pointers to objects whose resultant
    types correspond to conversion specifications given in the format
    specification.

    If no conversion specifications are given, you can omit these
    input pointers. Otherwise, the function call must have at least
    as many input pointers as there are conversion specifications,
    and the conversion specifications must match the types of the
    input pointers.

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