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.\" $FreeBSD: src/lib/libc/gen/fmtcheck.3,v 1.9 2004/07/02 23:52:10 ru 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 understand all of the conversions that
.Xr printf 3
does.