fmtcheck (3) - Linux Manuals

fmtcheck: sanitizes user-supplied


fmtcheck - sanitizes user-supplied

printf(3)Ns-style format string


Lb libbsd


In bsd/stdio.h Ft const char * Fn fmtcheck const char *fmt_suspect const char *fmt_default


The Fn fmtcheck scans Fa fmt_suspect and Fa fmt_default to determine if Fa fmt_suspect will consume the same argument types as Fa fmt_default and to ensure that Fa 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 Fn 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 Fa fmt_suspect is a valid format and consumes the same argument types as Fa fmt_default , then the Fn fmtcheck will return Fa fmt_suspect . Otherwise, it will return Fa fmt_default .


Note that the formats may be quite different as long as they accept the same arguments. For example, Qq Li %p %o %30s %#llx %-10.*e %n is compatible with Qq Li This number %lu %d%% and string %s has %qd numbers and %.*g floats (%n) . However, Qq Li %o is not equivalent to Qq Li %lx because the first requires an integer and the second requires a long.


The Fn fmtcheck function does not understand all of the conversions that printf(3) does.