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.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 16.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 17.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 18.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 19.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 20.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 21.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 22.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 23.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 24.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 25.\" POSSIBILITY OF SUCH DAMAGE. 26.\" 27.\" $FreeBSD$ 28.Dd October 16, 2002 29.Dt FMTCHECK 3 30.Os 31.Sh NAME 32.Nm fmtcheck 33.Nd sanitizes user-supplied 34.Xr printf 3 Ns -style 35format string 36.Sh LIBRARY 37.Lb libc 38.Sh SYNOPSIS 39.In stdio.h 40.Ft const char * 41.Fn fmtcheck "const char *fmt_suspect" "const char *fmt_default" 42.Sh DESCRIPTION 43The 44.Fn fmtcheck 45scans 46.Fa fmt_suspect 47and 48.Fa fmt_default 49to determine if 50.Fa fmt_suspect 51will consume the same argument types as 52.Fa fmt_default 53and to ensure that 54.Fa fmt_suspect 55is a valid format string. 56.Pp 57The 58.Xr printf 3 59family of functions cannot verify the types of arguments that they are 60passed at run-time. 61In some cases, like 62.Xr catgets 3 , 63it is useful or necessary to use a user-supplied format string with no 64guarantee that the format string matches the specified arguments. 65.Pp 66The 67.Fn fmtcheck 68was designed to be used in these cases, as in: 69.Bd -literal -offset indent 70printf(fmtcheck(user_format, standard_format), arg1, arg2); 71.Ed 72.Pp 73In the check, field widths, fillers, precisions, etc.\& are ignored (unless 74the field width or precision is an asterisk 75.Ql * 76instead of a digit string). 77Also, any text other than the format specifiers 78is completely ignored. 79.Sh RETURN VALUES 80If 81.Fa fmt_suspect 82is a valid format and consumes the same argument types as 83.Fa fmt_default , 84then the 85.Fn fmtcheck 86will return 87.Fa fmt_suspect . 88Otherwise, it will return 89.Fa fmt_default . 90.Sh SEE ALSO 91.Xr printf 3 92.Sh BUGS 93The 94.Fn fmtcheck 95function does not recognize positional parameters. 96.Sh SECURITY CONSIDERATIONS 97Note that the formats may be quite different as long as they accept the 98same arguments. 99For example, 100.Qq Li "%p %o %30s %#llx %-10.*e %n" 101is compatible with 102.Qq Li "This number %lu %d%% and string %s has %qd numbers and %.*g floats (%n)" . 103However, 104.Qq Li %o 105is not equivalent to 106.Qq Li %lx 107because 108the first requires an integer and the second requires a long. 109