Chars i C: signed
eller Unsigned??
Henry Spencer

There exist machunes (e.g. pdp11) on
which unsigned chars are a lot less efficient
than signed chars.

There exist machunes (e.g. ibm370) on
which signed chars are a lot less efficient than
unsigned chars.

Many applications do not care whether
the chars are signed or unsigned, so long as
they can be twiddled efficiently.

For this reason, char is intended to be the
more efficient of the two.

Many old programs assume that char is
signed; this does not make it so. Those pro-
grams are wrong, and have been all along.
Alas, this is not a comfort if you have to run
them.

The Father, the Son, and the Holy Ghost
(K&RI, HcS, and X3111 resp.) all agree that
characters in the "source character set"
(roughly, those one uses to write C) must
look positive. Actually, the Father and the
Son gave considerably broader guarantees,
but the Holy Ghost had to water them down a
bit.
The "unsigned char" type exists (in most
newer compilers) because there are a number
of situations where sign e%tension is very
awkward. For e%ample, getcharO wants to do
a non-sign%tended conversion from char to
int.

X3J11, in its semi-infinite wisdom, has
decided thu it would be nice to have a signed
counterpart to "unsigned char", to wit "signed
char". Therefore it is reasonable to expect
that most new compilers, and old ones
brought into conformance with the yet-to-be-
issued standard, will give you the full choice:
signed char if you rced signs, unsigned char
if you need everhing posidve, and char if
you don't care but want it to run fast.

Given that many cornpilers have qot yet
been upgraded to match even the current
X3J11 drafts, much kss the final endproduct
(which doesn't e%ist yet), any applicadon
which ces about signedness should use ty-
pedefs or macros for its char types, so that the
definidons can be revised lat«.

The only things you can safdy put into a
char variable, and depend on having them
come out unchanged, are characters from the
nadve character set and small positive ir,te-
gers.

Dennis Ritchie is on record, as I recall, as
saying that if he had to do it all over again, he
would consider changing his mind about
making chars signed on the pdpl I which is
how this rness got started). The pdp l l hard-
ware strongly encouraged this, but it has
caused a lot of trouble since. It is, however,
much too late to make such a cbange to C.

   .

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