Is there any way to get 64-bit time_t in 32-bit pr

问题:

On Windows I can call:

_time32(__time32_t); // to get 32-bit time_t
_time64(__time64_t); // to get 64-bit time_t

(both in 32 and 64-bit programs)

Is there any way do this in Linux (compiling with GCC)?

回答1:

Apparently, no it's not possible. For starters, there is only one time() function in Linux, no time32() or time64().

After searching for a while, I can see that it's not libc's fault, but the culprit is actually the kernel.

In order for libc to fetch the current time, it need to execute a system call for it: ^(Source)

time_t time (t) time_t *t;
{
    // ...
    INTERNAL_SYSCALL_DECL (err);
    time_t res = INTERNAL_SYSCALL (time, err, 1, NULL);
    // ...
    return res;
}

The system call is defined as: ^(Source)

SYSCALL_DEFINE1(time, time_t __user *, tloc)
{
    time_t i = get_seconds();
    // ...
    return i;
}

The function get_seconds() returns an unsigned long, like so: ^(Source)

unsigned long get_seconds(void)
{
    struct timekeeper *tk = &timekeeper;

    return tk->xtime_sec;
}

And timekeeper.xtime_sec is actually 64-bit: ^(Source)

struct timekeeper {
    // ...
    /* Current CLOCK_REALTIME time in seconds */
    u64                     xtime_sec;
    // ...
}

Now, if you know your C, you know that the size of unsigned long is actually implementation-dependant. On my 64-bit machine here, it's 64-bit; but on my 32-bit machine here, it's 32-bit. It possibly could be 64-bit on some 32-bit implementation, but there's no guarantee.

On the other hand, u64 is always 64-bit, so at the very base, the kernel keeps track of the time in a 64-bit type. Why it then proceeds to return this as an unsigned long, which is not guaranteed to be 64-bit long, is beyond me.

In the end, even if libc's would force time_t to hold a 64-bit value, it wouldn't change a thing.

You could tie your application deeply into the kernel, but I don't think it's even worth it.

回答2:

No time64()/time32() function are included into standard libraries.

No time32_t/time64_t defines are contemplated in standard headers.

time_t is defined into time.h as typedef __time_t time_t;

Following a long chain of redefines, you'll discover that __time_t is defined as 32 bit on 32 bit machines and 64bit on 64 bit machines.

回答3:

Many answers above said that this is impossible, but that's entirely incorrect. It was not possible at that time yet people had been talking about fixing it for years. Finally 64-bit time support on 32-bit platforms was introduced to the Linux 5.1 kernel with the addition of the new *time64 syscalls. Look at this table you can see that those syscalls aren't available on any 64-bit platforms. Now if you're writing code for 32-bit systems you can call clock_gettime64 directly (from inline assembly or C with syscall()) to get the current time

However after that you're completely on your own. If you want full userspace support you must be on Linux 5.6 or higher along with musl 1.2+ or glibc 2.32+. Just rebuild your code and your time_t will become 64-bit long

• All user space must be compiled with a 64-bit time_t, which will be supported in the coming musl-1.2 and glibc-2.32 releases, along with installed kernel headers from linux-5.6 or higher.

• Applications that use the system call interfaces directly need to be ported to use the time64 syscalls added in linux-5.1 in place of the existing system calls. This impacts most users of futex() and seccomp() as well as programming languages that have their own runtime environment not based on libc.

https://lkml.org/lkml/2020/1/29/355?anz=web

For more information

• Approaching the kernel year-2038 end game
• 64-bit time symbol handling in the GNU C Library
• glibc Y2038 Proofness Design

回答4:

Use this library: https://github.com/evalEmpire/y2038

The goal of this project is to provide a drop-in replacement for POSIX time.h which will work on machines which have a 32-bit time_t, yet not suffer from the 2038 bug. This will allow C programmers to be 2038-safe without having to rewrite their software to a new interface. It does this while still using the system time zone database.

回答5:

If you really need this, why not roll your own?

typedef int32_t my_time32;
typedef int64_t my_time64;


my_time32 get_mytime32() {
    if (sizeof(time_t) == sizeof(my_time32))
        return time(NULL);
    else {
        /* Check for overflow etc. here... */
        return (my_time32)(time(NULL));
    }
}

And similarly for get_mytime64().

If you do not care about overflow, a simple return time(NULL); would work for both functions thanks to C's implicit numeric conversions.