Why the UNIX time(2) system call is designed like that?

Question

The UNIX time(2) system call, time_t time(time_t *t);, returns the current time in two ways: return value and return-by-reference. What's the rational for this redundancy? Why not just define it time_t time(void);?

Answer 1

Ancient history, but it is probably related to the time when a long was simulated by two 16-bit int values in tandem. Otherwise, there's no real obvious reason. Interestingly, the UNIX™ 7th Edition Manual documents time as obsolete:

NAME

time, ftime – get date and time

SYNOPSIS

long time(0)
long time(tloc)
long *tloc;

#include <sys/types.h>
#include <sys/timeb.h>
ftime(tp)
struct timeb *tp;

DESCRIPTION

Time returns the time since 00:00:00 GMT, Jan. 1, 1970, measured in seconds. If tloc is nonnull, the return value is also stored in the place to which tloc points. The ftime entry fills in a structure pointed to by its argument, as defined by <sys/timeb.h>:

/*
* Structure returned by ftime system call
*/
struct timeb
{
time_t time;
unsigned short millitm;
short timezone;
short dstflag;
};

The structure contains the time since the epoch in seconds, up to 1000 milliseconds of more-precise interval, the local timezone (measured in minutes of time westward from Greenwich), and a flag that, if nonzero, indicates that Daylight Saving time applies locally during the appropriate part of the year.

SEE ALSO

date(1), stime(2), ctime(3)

ASSEMBLER

(ftime = 35.)
sys ftime; bufptr

(time = 13.; obsolete call)
sys time
(time since 1970 in r0-r1)

Note the designation 'obsolete call', and note that the return value was in two (16-bit) registers, r0 and r1.