bin/145748: hexdump(1) %s format qualifier broken

Bruce Evans brde at
Wed Apr 21 16:50:24 UTC 2010

On Wed, 21 Apr 2010, Garrett Cooper wrote:

> On Tue, Apr 20, 2010 at 7:33 AM, Wayne Sierke <ws at> wrote:
> >> The fact that "%4s" fails and isn't noted in the addendum is a failure
> >> according to the specifications of hexdump as per the manpage; "%.4s"
> >> passing is a reasonable workaround for broken "%[:digit:]s"
> >> functionality.
> >
> > I should have made my earlier reply more explicit. It doesn't seem to be
> > a failure.
> The issue with %4s failing is still a failure. The non-issue with
> %.4s, %0.4s etc not failing is not a failure; it's just a bit more
> obfuscated logic.

The behaviour is as documented.  %4s is an invalid format since it has
a field width but no precision.  %.4s is a valid format since it has a

> > The part of the hexdump(1) manpage quoted previously:
> >
> > o A byte count or field precision is required for each ``s'' con-
> > version character (unlike the fprintf(3) default which prints
> > the entire string if the precision is unspecified).
> That statement is misleading. It should make the above statement with
> field width, not [field] precision. FWIW, the statement `field
> precision' makes absolutely no sense in the terminology used by
> printf(3), and is most likely a typo.

Nothing misleading there.  The man page should and does match the code,
which takes a field precision.  The statement `field precision' exactly
matches printf(3) terminology.

I think the field precision, if any, is supposed to be silently ignored,
and the man page doesn't say enough about that, and the code may have
bugs with it, causing the present confusion.  I haven't checked exactly
why hexdump uses the precision and not the field width, but this
behaviour makes some sense.  Use of the field width would pad the
string, while use of the precision clips it, and hexdump apparently
only supports the latter.

> And finally, yes I agree that %s is illegal because you can't qualify
> the number of characters required for each format unit -- something
> that's required for hexdump to function. %4s, etc with precision not
> being specified is legal however.

%4s doesn't have any precision.  I think %<something>s is supposed to
be legal if there is a precision or a byte count.  However, without
these, silently ignoring the field width in %4s reduces it to %s, so
it should cause the same error as %s.

> > And as observed hexdump does accept the required value when passed a
> > "field precision" - the numeric value immediately after the period in
> > "%.4s" (NB not a "field width" - as described in fprintf(3) and slightly
> > more clearly in printf(3)).
> From printf(3):
>      o   An optional decimal digit string specifying a minimum field width.
>          If the converted value has fewer characters than the field width, it
>          will be padded with spaces on the left (or right, if the left-adjust-
>          ment flag has been given) to fill out the field width.
>      o   An optional precision, in the form of a period . followed by an
>          optional digit string.  If the digit string is omitted, the precision
>          is taken as zero.  This gives the minimum number of digits to appear
>          for d, i, o, u, x, and X conversions, the number of digits to appear
>          after the decimal-point for a, A, e, E, f, and F conversions, the
>          maximum number of significant digits for g and G conversions, or the
>          maximum number of characters to be printed from a string for s con-
>          versions.
> Note the word `optional' in the first and second clauses. `.' isn't
> required except to disambiguate precision from field width.

The "." is part of the syntax for a precision, so it is required to specify
a precision.


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