FMTCHECK(3)            FreeBSD Library Functions Manual            FMTCHECK(3)

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     const char *
     fmtcheck(const char *fmt_suspect, const char *fmt_default);

DESCRIPTION
     The fmtcheck() scans fmt_suspect and fmt_default to determine if
     fmt_suspect will consume the same argument types as fmt_default and to
     ensure that fmt_suspect 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
     guarantee that the format string matches the specified arguments.

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

           printf(fmtcheck(user_format, standard_format), 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 fmt_suspect is a valid format and consumes the same argument types as
     fmt_default, then the fmtcheck() will return fmt_suspect.  Otherwise, it
     will return fmt_default.

SEE ALSO
     printf(3)

BUGS
     The fmtcheck() function does not recognize positional parameters.

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.

FreeBSD 13.1-RELEASE-p6        October 16, 2002        FreeBSD 13.1-RELEASE-p6