The fmtcheck scans fmt_suspect and fmt_default to determine if fmt_suspect will consume the same argument types as fmt_default and to ensure that fmt_suspect is a valid format string.
The printf(3) family of functions cannot verify the types of arguments that they are passed at run-time. In some cases, like catgets(3), it is useful or necessary to use a user-supplied format string with no guarantee that the format string matches the specified arguments.
The fmtcheck was designed to be used in these cases, as in:
printf(fmtcheck(user_format, standard_format), arg1, arg2);
In the check, field widths, fillers, precisions, etc. are ignored (unless the field width or precision is an asterisk * instead of a digit string). Also, any text other than the format specifiers is completely ignored.
If fmt_suspect is a valid format and consumes the same argument types as fmt_default, then the fmtcheck will return fmt_suspect. Otherwise, it will return fmt_default.
Note that the formats may be quite different as long as they accept the same arguments. For example, ""%p %o %30s %#llx %-10.*e %n"" is compatible with ""This number %lu %d%% and string %s has %qd numbers and %.*g floats (%n)"". However, "%o" is not equivalent to "%lx" because the first requires an integer and the second requires a long.