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.\" $FreeBSD: release/9.1.0/lib/libc/gen/fmtcheck.3 208027 2010-05-13 12:07:55Z uqs $
.Dd October 16, 2002
.Dt FMTCHECK 3
.Os
.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 SEE ALSO
.Xr printf 3
.Sh BUGS
The
.Fn fmtcheck
function does not recognize positional parameters.
.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.