118 lines
3.7 KiB
Groff
118 lines
3.7 KiB
Groff
.\" Copyright (c) 2000 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This file was contributed to The NetBSD Foundation by Allen Briggs.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.Dd October 16, 2002
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.Os
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.Dt FMTCHECK 3
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.Sh NAME
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.Nm fmtcheck
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.Nd sanitizes user-supplied
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.Xr printf 3 Ns -style
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format string
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdio.h
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.Ft const char *
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.Fn fmtcheck "const char *fmt_suspect" "const char *fmt_default"
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.Sh DESCRIPTION
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The
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.Fn fmtcheck
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scans
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.Fa fmt_suspect
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and
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.Fa fmt_default
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to determine if
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.Fa fmt_suspect
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will consume the same argument types as
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.Fa fmt_default
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and to ensure that
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.Fa fmt_suspect
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is a valid format string.
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.Pp
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The
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.Xr printf 3
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family of functions cannot verify the types of arguments that they are
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passed at run-time.
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In some cases, like
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.Xr catgets 3 ,
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it is useful or necessary to use a user-supplied format string with no
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guarantee that the format string matches the specified arguments.
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.Pp
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The
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.Fn fmtcheck
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was designed to be used in these cases, as in:
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.Bd -literal -offset indent
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printf(fmtcheck(user_format, standard_format), arg1, arg2);
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.Ed
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.Pp
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In the check, field widths, fillers, precisions, etc.\& are ignored (unless
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the field width or precision is an asterisk
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.Ql *
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instead of a digit string).
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Also, any text other than the format specifiers
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is completely ignored.
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.Sh RETURN VALUES
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If
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.Fa fmt_suspect
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is a valid format and consumes the same argument types as
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.Fa fmt_default ,
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then the
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.Fn fmtcheck
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will return
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.Fa fmt_suspect .
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Otherwise, it will return
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.Fa fmt_default .
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.Sh SECURITY CONSIDERATIONS
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Note that the formats may be quite different as long as they accept the
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same arguments.
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For example,
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.Qq Li "%p %o %30s %#llx %-10.*e %n"
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is compatible with
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.Qq Li "This number %lu %d%% and string %s has %qd numbers and %.*g floats (%n)" .
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However,
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.Qq Li %o
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is not equivalent to
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.Qq Li %lx
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because
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the first requires an integer and the second requires a long.
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.Sh SEE ALSO
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.Xr printf 3
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.Sh BUGS
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The
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.Fn fmtcheck
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function does not understand all of the conversions that
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.Xr printf 3
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does.
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