how to use settimeofday(2)?

Question

What am I doing wrong here? I expect settimeofday() to change the system time, not return EINVAL.

$ uname -a
Linux io 4.3.5-300.fc23.x86_64 #1 SMP Mon Feb 1 03:18:41 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
$ cat settimeofday.c 
#include <sys/time.h>
#include <stdio.h>

int main()
{
    struct timeval tv = {0, 0};
    if (settimeofday(&tv, 0) == -1)
        perror("settimeofday");
}
$ gcc settimeofday.c 
$ sudo ./a.out 
settimeofday: Invalid argument

The error is coming from a Thinkpad T450 running Fedora 23. The same code runs fine on OS X.

EDIT

To clarify, the command is being executed as root:

# whoami
root
# sudo ./a.out 
settimeofday: Invalid argument

As expected, I get EPERM not EINVAL if I run the program as a regular user:

$ ./a.out 
settimeofday: Operation not permitted

Note that you need permission to `settimeofday`, not that it should return an `EINVAL` error if you're not running as root. — Colonel Thirty Two, Feb 12 '16 at 02:31
you should be sudo user or root user, by default its not allowed for normal user to change the time — Dilip Kumar, Feb 12 '16 at 02:33
See the "sudo" in "sudo ./a.out". The command is being run as root. — Josh Johnson, Feb 12 '16 at 02:34
On some systems, under certain security settings, even the superuser can not set the time backwards with `settimeofday()` -- is this something Fedora does? — John Hascall, Feb 12 '16 at 04:46
No. The call fails with EINVAL no matter what the timeval is set to, even if would cause time to jump forward. Anyways, if this was a security issue then I would expect EPERM. — Josh Johnson, Feb 12 '16 at 04:58
You don't have a Linux Security Module installed that hooks into the settime syscall do you? — John Hascall, Feb 12 '16 at 05:18
So I was wrong when I commented earlier and said that the call fails no matter what the timeval is set to. See my answer for details. — Josh Johnson, Feb 12 '16 at 06:10

score 5 · Accepted Answer · answered Feb 12 '16 at 06:27

Commit e1d7ba was introduced to the Linux kernel in mid-2015 and restricts the value of the tv_sec field. The restriction is influenced by system uptime -- see the commit message and related LKML discussion for details.

That's what was causing the settimeofday call to return EINVAL and explains why the code runs on OS X and older Linux machines.

score 0 · Answer 2 · answered Feb 12 '16 at 03:54

0

As shown in the manpage of settimeofday().

If either tv or tz is NULL, the corresponding structure is not set or returned.

answered Feb 12 '16 at 03:54

gzh

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And a few lines further down it says: `The use of the timezone structure is obsolete; the tz argument should normally be specified as NULL.` – John Hascall Feb 12 '16 at 04:39
@John Hascall, After fully read the manpage of setimeofday(), it seems that the tz_dsttime fied of timezone will not be supported in linux, but the tz_minuteswest filed is still needed to be set. I am not sure I read correctly. – gzh Feb 12 '16 at 05:01
Numerous reputable examples (such as Robert Love's Linux System Programming) show settimeofday() used with the second argument set to NULL. Furthermore, I tried passing in a non-NULL timezone struct and it made no difference. – Josh Johnson Feb 12 '16 at 05:06
It seems you are prohibited to set tv to {0, 0}, the start of Epoch time. as mentioned in [this page](http://www.gnu.org/software/libc/manual/html_node/High_002dResolution-Calendar.html), _Use adjtime (below) to make a smooth transition_ @JoshuaJohnson – gzh Feb 12 '16 at 05:22
1

The consequence of set tv to {0, 0}: http://www.wired.com/2016/02/dont-set-your-iphone-back-to-1970-no-matter-what/ – gzh Feb 16 '16 at 12:14

how to use settimeofday(2)?

2 Answers2

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