The printf(3) family of functions can not 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 parameters.
The function 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.
Note that the formats may be quite different as long as they accept the same parameters. For example, "%ld %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, and "%p" is not equivalent to "%lu" because the first requires a pointer and the second requires a long.