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.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 n 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.
p 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.
p The .Fn fmtcheck was designed to be used in these cases, as in: d -literal -offset indent printf(fmtcheck(user_format, standard_format), arg1, arg2); .Ed
p 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.