NAME

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

LIBRARY

library “libbsd”

SYNOPSIS

#include <stdio.h> (See libbsd(7) for include usage.)
const char *
fmtcheck(const char *fmt_suspect, const char *fmt_default);

DESCRIPTION

The fmtcheck function 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 can not 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 parameters.

The fmtcheck function 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.

Note that the formats may be quite different as long as they accept the same parameters. For example, "%ld %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, and "%p" is not equivalent to "%lu" because the first requires a pointer and the second requires a long.

RETURN VALUES

If fmt_suspect is a valid format and consumes the same argument types as fmt_default, then the fmtcheck function will return fmt_suspect. Otherwise, it will return fmt_default.

SEE ALSO

printf(3)