Set a clock
Synopsis:
#include <time.h>
int clock_settime( clockid_t id,
const struct timespec * tp );
Arguments:
- id
- CLOCK_REALTIME, the ID of the clock that maintains the system time.
For more information about the different clocks, see
"Other clock sources"
in the Clocks, Timers, and Getting a Kick Every So Often of
Getting Started with QNX Neutrino.
- tp
- A pointer to a
timespec
structure containing at least the following members:
- tv_sec — the number of seconds since 1970.
- tv_nsec — the number of nanoseconds in the current second.
This value increases by some multiple of nanoseconds, based on the system clock's resolution.
Library:
libc
Use the -l c option to
qcc
to link against this library.
This library is usually included automatically.
Description:
The clock_settime() function sets the clock specified by
id to the time specified in the buffer pointed to by
tp.
Note:
- Be careful if you set the date during the period that a time zone is switching from
daylight saving time (DST) to standard time.
When a time zone changes to standard time, the local time goes back one hour
(for example, 2:00 a.m. becomes 1:00 a.m.).
The local time during this hour is ambiguous (e.g., 1:14 a.m. occurs twice
in the morning that the time zone switches to standard time).
To avoid problems, use UTC time to set the date in this period.
- You can't set the time when the id is CLOCK_MONOTONIC.
- In order to set the clock, your process must have the
PROCMGR_AID_CLOCKSET ability enabled.
For more information, see
procmgr_ability().
Returns:
- 0
- Success
- -1
- An error occurred
(errno
is set).
Errors:
- EINVAL
- One of the following occurred:
- The id is invalid.
You can't set the time for CLOCK_MONOTONIC
or for a process or thread CPU-time clock.
- The number of nanoseconds specified
by the tv_nsec member is less than zero or greater than
or equal to 1000 million.
- The tv_sec member is -1 (which could happen if
you set it to the result from
mktime()
without checking to see if the call succeeded).
- EPERM
- The calling process doesn't have the required permission; see
procmgr_ability().
Examples:
/* This program sets the clock forward 1 day. */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
int main( void )
{
struct timespec stime;
if( clock_gettime( CLOCK_REALTIME, &stime) == -1 ) {
perror( "getclock" );
return EXIT_FAILURE;
}
stime.tv_sec += (60*60)*24L; /* Add one day */
stime.tv_nsec = 0;
if( clock_settime( CLOCK_REALTIME, &stime) == -1 ) {
perror( "setclock" );
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
Classification:
POSIX 1003.1
Safety: |
|
Cancellation point |
No |
Interrupt handler |
No |
Signal handler |
Yes |
Thread |
Yes |