FMTCHECK(3)              BSD Library Functions Manual              FMTCHECK(3)

NAME
     fmtcheck -- sanitizes user-supplied printf(3)-style format string

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     ##include <>

     const char *
     fmtcheck(const char *fmtsuspect, const char *fmtdefault);

DESCRIPTION
     The fmtcheck() scans fmtsuspect and fmtdefault to determine if
     fmtsuspect will consume the same argument types as fmtdefault and to
     ensure that fmtsuspect is a valid format string.

     The printf(3) family of functions cannot verify the types of arguments
     that they are passed at run-time.  In some cases, like catgets(3), it is
     useful or necessary to use a user-supplied format string with no guaran-
     tee that the format string matches the specified arguments.

     The fmtcheck() was designed to be used in these cases, as in:

           printf(fmtcheck(userformat, standardformat), arg1, arg2);

     In the check, field widths, fillers, precisions, etc. are ignored (unless
     the field width or precision is an asterisk `*' instead of a digit
     string).  Also, any text other than the format specifiers is completely
     ignored.

RETURN VALUES
     If fmtsuspect is a valid format and consumes the same argument types as
     fmtdefault, then the fmtcheck() will return fmtsuspect.  Otherwise, it
     will return fmtdefault.

SECURITY CONSIDERATIONS
     Note that the formats may be quite different as long as they accept the
     same arguments.  For example, "%p %o %30s %#llx %-10.*e %n" is compatible
     with "This number %lu %d%% and string %s has %qd numbers and %.*g floats
     (%n)".  However, "%o" is not equivalent to "%lx" because the first
     requires an integer and the second requires a long.

SEE ALSO
     printf(3)

BUGS
     The fmtcheck() function does not understand all of the conversions that
     printf(3) does.

BSD                            October 16, 2002                            BSD