.\" Copyright (c) 2000 The NetBSD Foundation, Inc. .\" All rights reserved. .\" .\" This file was contributed to The NetBSD Foundation by Allen Briggs. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the NetBSD .\" Foundation, Inc. and its contributors. .\" 4. Neither the name of The NetBSD Foundation nor the names of its .\" contributors may be used to endorse or promote products derived .\" from this software without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS .\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED .\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS .\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR .\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF .\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS .\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN .\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE .\" POSSIBILITY OF SUCH DAMAGE. .\" .\" $FreeBSD: src/lib/libc/gen/fmtcheck.3,v 1.10 2008/08/02 06:02:42 das Exp $ .Dd October 16, 2002 .Os .Dt FMTCHECK 3 .Sh NAME .Nm fmtcheck .Nd sanitizes user-supplied .Xr printf 3 Ns -style format string .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In stdio.h .Ft const char * .Fn fmtcheck "const char *fmt_suspect" "const char *fmt_default" .Sh DESCRIPTION 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. .Pp The .Xr printf 3 family of functions cannot verify the types of arguments that they are passed at run-time. In some cases, like .Xr 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. .Pp The .Fn fmtcheck was designed to be used in these cases, as in: .Bd -literal -offset indent printf(fmtcheck(user_format, standard_format), arg1, arg2); .Ed .Pp In the check, field widths, fillers, precisions, etc.\& are ignored (unless the field width or precision is an asterisk .Ql * instead of a digit string). Also, any text other than the format specifiers is completely ignored. .Sh RETURN VALUES 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 . .Sh SECURITY CONSIDERATIONS 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. .Sh SEE ALSO .Xr printf 3 .Sh BUGS The .Fn fmtcheck function does not recognize positional parameters.