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.

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.