FMTCHECK(3) | FreeBSD Library Functions Manual | FMTCHECK(3) |
NAME
fmtcheck — sanitizes user-supplied printf(3)-style format stringLIBRARY
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.
October 16, 2002 | FreeBSD |