Why does `gmtime` take a pointer?

Question

According to the documentation the struct tm *gmtime(const time_t *timer); is supposed to convert the time_t pointed to by timer to a broken down time.

Now is there a reason why they decided to make the function take a pointer to the time_t instead of passing the time_t directly?

As far as I can see time_t is of arithmetic type and should therefore have been possible to pass directly (also I find it reasonable that it would have fit into a long). Also there seem to be any specific handling of the NULL pointer (which could have motivated passing a pointer).

Is there something I'm missing? Something still relevant today?

Answer 1

From what I've seen, it's more a historical quirk. When time.h was first introduced, and with it functions like time, it used a value that could not be returned (ie: no long int etc). The standard defined an obscure time_t type that still leaves a lot of room for vendors to implement in weird ways (it has to be an arithmetic type, but no ranges or max values are defined for example) - C11 standard:

7.27.1 Components of time.
[...]
3 .The types declared are size_t (described in 7.19);
clock_t
and
time_t
which are real types capable of representing times;
4. The range and precision of times representable in clock_t and time_t are implementation-defined.

size_t is described, in C11, as "is the unsigned integer type of the result of the sizeof operator;"

In light of this, your comment ("I find it reasonable that it would have fit into a long") is an understandable one, but it's incorrect, or at the very least inaccurate. POSIX, for example requires time_t to be an integer or real floating type. A long fits that description, but so would a long double which doesn't fit in a long. A more accurate assumption to make would be that the minimum size of time_tis a 32bit int (until 2038 at least), but that time_t preferably is a 64bit type.

Anyway, back in those days, if a value couldn't be returned, the only alternative was to pass the memory to the function (which is a sensible thing to do).
This is why we have functions like

time_t time(time_t *t);

It doesn't really make sense to set the same value twice: once by returning it, and once using indirection, but the argument is there because originally, the function was defined as

time(time_t *t)

Note the lack of a return type, if time were added today, it'd either be defined as void time( time_t * ), or if the committee hadn't been drinking, and realized the absurdity of passing a pointer here, they'd define it as time_t time ( void );

Looking at the C11 standard regarding the time function, it seems that the emphasis of the functions' behaviour is on the return value. The pointer argument is mentioned briefly but it certainly isn't given any significance:

7.27.2.4 The time function

1. Synopsis

#include <time.h>
time_t time(time_t *timer);

2. Description

The time function determines the current calendar time. The encoding of the value is unspecified.

3. Returns

The time function returns the implementation’s best approximation to the current calendar time. The value (time_t)(-1) is returned if the calendar time is not available. If timer is not a null pointer, the return value is also assigned to the object it points to.

The main thing we can take from this is that, as far as the standard goes, the pointer is just a secondary way of getting at the return value of the function. Given that the return value indicates something went wrong ((time_t)(-1)), I'd argue that we should treat this function as though it was meant to be time_t time( void ).

But because the old implementation is still kicking around, and we've all gotten used to it, it's one of those things that should've been marked for deprecation, but because it never really was, it probably will be part of the C language for ever...

The only other reason why functions use time_t like this (const) is either historical, or to maintain a consistent API across the time.h API. AFAIK that is

TL;DR

Most time.h function use pointers to the time_t type for historical, compatibility, and consistency reasons.

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I knew I read that stuff about the early days of the time function before, here's a related SO answer