.\" 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.
.\"
.\" $FreeBSD$
.Dd October 17, 2000
.Os
.Dt FMTCHECK 3
.Sh NAME
.Nm fmtcheck
.Nd sanitizes user-supplied printf(3)-style format string
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <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 can not 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 parameters.
.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.
.Pp
Note that the formats may be quite different as long as they accept the
same parameters.  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 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 SEE ALSO
.Xr printf 3