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FMTCHECK(3)              BSD 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() 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. runtime.
     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 com-pletely completely
     pletely ignored.

RETURN VALUES
     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.

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
     The fmtcheck() function does not understand all of the conversions that printf(3) does.

BSD                            October 16, 2002                            BSD