NAME
fmtcheck - sanitizes user-supplied printf(3)-style format string
LIBRARY
Utility functions from BSD systems (libbsd, -lbsd)
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.
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.