fmtcheck man page on FreeBSD

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FMTCHECK(3)		 BSD Library Functions Manual		   FMTCHECK(3)

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

     Standard C Library (libc, -lc)

     #include <stdio.h>

     const char *
     fmtcheck(const char *fmt_suspect, const char *fmt_default);

     The fmtcheck() 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 guaran‐
     tee 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

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

     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.


     The fmtcheck() function does not recognize positional parameters.

BSD			       October 16, 2002				   BSD

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