fmtcheck(3) - NetBSD Manual Pages

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FMTCHECK(3)             NetBSD Library Functions Manual            FMTCHECK(3)


NAME

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


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <stdio.h>

     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)

NetBSD 10.1                      June 14, 2014                     NetBSD 10.1