fmtcheck(3bsd) LOCAL fmtcheck(3bsd)
NAME
fmtcheck — sanitizes user-supplied printf(3)-style format string
LIBRARY
Utility functions from BSD systems (libbsd, -lbsd)
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 guaran-
tee 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 compati-
ble 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. Other-
wise, it will return fmt_default.
SEE ALSO
printf(3)
BSD June 14, 2014 BSD
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