1.\" Copyright (c) 2000 The NetBSD Foundation, Inc. 2.\" All rights reserved. 3.\" 4.\" This file was contributed to The NetBSD Foundation by Allen Briggs. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the NetBSD 17.\" Foundation, Inc. and its contributors. 18.\" 4. Neither the name of The NetBSD Foundation nor the names of its 19.\" contributors may be used to endorse or promote products derived 20.\" from this software without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 23.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 24.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 25.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 26.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 27.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 28.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 29.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 30.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 31.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 32.\" POSSIBILITY OF SUCH DAMAGE. 33.\" 34.\" $FreeBSD$ 35.Dd October 16, 2002 36.Os 37.Dt FMTCHECK 3 38.Sh NAME 39.Nm fmtcheck 40.Nd sanitizes user-supplied 41.Xr printf 3 Ns -style 42format string 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.In stdio.h 47.Ft const char * 48.Fn fmtcheck "const char *fmt_suspect" "const char *fmt_default" 49.Sh DESCRIPTION 50The 51.Fn fmtcheck 52scans 53.Fa fmt_suspect 54and 55.Fa fmt_default 56to determine if 57.Fa fmt_suspect 58will consume the same argument types as 59.Fa fmt_default 60and to ensure that 61.Fa fmt_suspect 62is a valid format string. 63.Pp 64The 65.Xr printf 3 66family of functions cannot verify the types of arguments that they are 67passed at run-time. 68In some cases, like 69.Xr catgets 3 , 70it is useful or necessary to use a user-supplied format string with no 71guarantee that the format string matches the specified arguments. 72.Pp 73The 74.Fn fmtcheck 75was designed to be used in these cases, as in: 76.Bd -literal -offset indent 77printf(fmtcheck(user_format, standard_format), arg1, arg2); 78.Ed 79.Pp 80In the check, field widths, fillers, precisions, etc.\& are ignored (unless 81the field width or precision is an asterisk 82.Ql * 83instead of a digit string). 84Also, any text other than the format specifiers 85is completely ignored. 86.Sh RETURN VALUES 87If 88.Fa fmt_suspect 89is a valid format and consumes the same argument types as 90.Fa fmt_default , 91then the 92.Fn fmtcheck 93will return 94.Fa fmt_suspect . 95Otherwise, it will return 96.Fa fmt_default . 97.Sh SECURITY CONSIDERATIONS 98Note that the formats may be quite different as long as they accept the 99same arguments. 100For example, 101.Qq Li "%p %o %30s %#llx %-10.*e %n" 102is compatible with 103.Qq Li "This number %lu %d%% and string %s has %qd numbers and %.*g floats (%n)" . 104However, 105.Qq Li %o 106is not equivalent to 107.Qq Li %lx 108because 109the first requires an integer and the second requires a long. 110.Sh SEE ALSO 111.Xr printf 3 112.Sh BUGS 113The 114.Fn fmtcheck 115function does not understand all of the conversions that 116.Xr printf 3 117does. 118