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FMTCHECK(3) NetBSD Library Functions Manual FMTCHECK(3)
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fmtcheck -- sanitizes user-supplied printf(3)-style format string
Standard C Library (libc, -lc)
const char *
fmtcheck(const char *fmt_suspect, const char *fmt_default);
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
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.
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.
NetBSD 10.99 June 14, 2014 NetBSD 10.99