Newsgroup: comp.lang.c

Keith Thompson wrote:
> August Karlstrom <fusionfive@comhem.se> writes:
>> Diomidis Spinellis wrote:
>>> August Karlstrom wrote:
>>>
>>>> Does anyone know why some of the functions in time.h use pointers
>>>> to constant objects of type time_t when time_t is an aritmetic
>>>> type. Why is e.g. ctime declared as
>>>>
>>>>    char* ctime(const time_t* tp);
>>>>
>>>> and not as
>>>>
>>>>    char* ctime(time_t t);
>>> My guess is that this is an implementation decision related to the
>>> environment where C has its roots.  In the Seventh Edition Unix (and
>>> probably also in earlier versions), time_t is implemented as a long
>>> <http://minnie.tuhs.org/UnixTree/V7/usr/include/sys/types.h.html>.
>>> On a PDP-11 where that 1979 version of Unix run, passing as an
>>> argument a 16 bit pointer to a 32 bit long was probably more
>>> efficient than passing the actual 32 bit value.
>>>
[...]
> I *think* that on some early implementations, 32-bit ints weren't
> directly supported, and what's now a ftime_t was probably defined as
> an array of two ints.  The declaration of time() may have been
> something like:
> 
>     time(int t[2]);

You are absolutely right.  In the Third Edition Unix (February 1973) the 
time(2) interface is specified in assembly language: the 32-bit result 
is returned in the register pair r0/r1 
<http://minnie.tuhs.org/UnixTree/V3/usr/man/man2/time.2.html>.  In the 
Fourth Edition Unix (November 1973), which was (re)written in C, time 
takes as an argument an int tvec[2] 
<http://minnie.tuhs.org/UnixTree/V4/usr/man/man2/time.2.html>.  Ritchie 
mentions that the long type was added to C during the period 1973-1980 
<http://cm.bell-labs.com/cm/cs/who/dmr/chist.html>.  The long type 
certainly wasn't supported by the C compiler that came with the Fifth 
Edition Unix that was released on June 1974 
<http://minnie.tuhs.org/UnixTree/V5/usr/c/c00.c.html>.  Therefore, when 
time(2) was first specified in C, an int[2] argument was a reasonable 
interface specification.

-- 
Diomidis Spinellis
Code Quality: The Open Source Perspective (Addison-Wesley 2006)
http://www.spinellis.gr/codequality?clc

Unless otherwise expressly stated, all original material on this page created by Diomidis Spinellis is licensed under a Creative Commons Attribution-Share Alike 3.0 Greece License.