table of contents
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FMTCHECK(3) | Library Functions Manual | FMTCHECK(3) |
NAME¶
fmtcheck
—
LIBRARY¶
library “libbsd”SYNOPSIS¶
#include <bsd/stdio.h>
const char *
fmtcheck
(const
char *fmt_suspect, const
char *fmt_default);
DESCRIPTION¶
Thefmtcheck
() 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 thefmtcheck
() 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¶
Thefmtcheck
() function does not understand all of the
conversions that printf(3) does.
October 16, 2002 | Linux 4.9.0-9-amd64 |