9. Clock Manager — RTEMS Classic API Guide 5.4-rc2 (21st August 2026) documentation

# 9. Clock Manager

## 9.1. Introduction

The clock manager provides support for time of day and other time related capabilities. The directives provided by the clock manager are:

- rtems_clock_set - Set date and time

- rtems_clock_get_tod - Get date and time in TOD format

- rtems_clock_get_tod_timeval - Get date and time in timeval format

- rtems_clock_get_seconds_since_epoch - Get seconds since epoch

- rtems_clock_get_ticks_per_second - Get ticks per second

- rtems_clock_get_ticks_since_boot - Get current ticks counter value

- rtems_clock_tick_later - Get tick value in the future

- rtems_clock_tick_later_usec - Get tick value in the future in microseconds

- rtems_clock_tick_before - Is tick value is before a point in time

- rtems_clock_get_uptime - Get time since boot

- rtems_clock_get_uptime_timeval - Get time since boot in timeval format

- rtems_clock_get_uptime_seconds - Get seconds since boot

- rtems_clock_get_uptime_nanoseconds - Get nanoseconds since boot

## 9.2. Background

### 9.2.1. Required Support

For the features provided by the clock manager to be utilized, periodic timer interrupts are required. Therefore, a real-time clock or hardware timer is necessary to create the timer interrupts. The clock tick directive is normally called by the timer ISR to announce to RTEMS that a system clock tick has occurred. Elapsed time is measured in ticks. A tick is defined to be an integral number of microseconds which is specified by the user in the Configuration Table.

### 9.2.2. Time and Date Data Structures

The clock facilities of the clock manager operate upon calendar time. These directives utilize the following date and time structure for the native time and date format:

    struct rtems_tod_control {
        uint32_t year;   /* greater than 1987 */
        uint32_t month;  /* 1 - 12 */
        uint32_t day;    /* 1 - 31 */
        uint32_t hour;   /* 0 - 23 */
        uint32_t minute; /* 0 - 59 */
        uint32_t second; /* 0 - 59 */
        uint32_t ticks;  /* elapsed between seconds */
    };
    typedef struct rtems_tod_control rtems_time_of_day;

The native date and time format is the only format supported when setting the system date and time using the `rtems_clock_set` directive. Some applications expect to operate on a UNIX-style date and time data structure. The `rtems_clock_get_tod_timeval` always returns the date and time in `struct timeval` format.

The `struct timeval` data structure has two fields: `tv_sec` and `tv_usec` which are seconds and microseconds, respectively. The `tv_sec` field in this data structure is the number of seconds since the POSIX epoch of January 1, 1970 but will never be prior to the RTEMS epoch of January 1, 1988.

### 9.2.3. Clock Tick and Timeslicing

Timeslicing is a task scheduling discipline in which tasks of equal priority are executed for a specific period of time before control of the CPU is passed to another task. It is also sometimes referred to as the automatic round-robin scheduling algorithm. The length of time allocated to each task is known as the quantum or timeslice.

The system’s timeslice is defined as an integral number of ticks, and is specified in the Configuration Table. The timeslice is defined for the entire system of tasks, but timeslicing is enabled and disabled on a per task basis.

The clock tick directives implement timeslicing by decrementing the running task’s time-remaining counter when both timeslicing and preemption are enabled. If the task’s timeslice has expired, then that task will be preempted if there exists a ready task of equal priority.

### 9.2.4. Delays

A sleep timer allows a task to delay for a given interval or up until a given time, and then wake and continue execution. This type of timer is created automatically by the `rtems_task_wake_after` and `rtems_task_wake_when` directives and, as a result, does not have an RTEMS ID. Once activated, a sleep timer cannot be explicitly deleted. Each task may activate one and only one sleep timer at a time.

### 9.2.5. Timeouts

Timeouts are a special type of timer automatically created when the timeout option is used on the `rtems_message_queue_receive`, `rtems_event_receive`, `rtems_semaphore_obtain` and `rtems_region_get_segment` directives. Each task may have one and only one timeout active at a time. When a timeout expires, it unblocks the task with a timeout status code.

## 9.3. Operations

### 9.3.1. Announcing a Tick

RTEMS provides the several clock tick directives which are called from the user’s real-time clock ISR to inform RTEMS that a tick has elapsed. Depending on the timer hardware capabilities the clock driver must choose the most appropriate clock tick directive. The tick frequency value, defined in microseconds, is a configuration parameter found in the Configuration Table. RTEMS divides one million microseconds (one second) by the number of microseconds per tick to determine the number of calls to the clock tick directive per second. The frequency of clock tick calls determines the resolution (granularity) for all time dependent RTEMS actions. For example, calling the clock tick directive ten times per second yields a higher resolution than calling the clock tick two times per second. The clock tick directives are responsible for maintaining both calendar time and the dynamic set of timers.

### 9.3.2. Setting the Time

The `rtems_clock_set` directive allows a task or an ISR to set the date and time maintained by RTEMS. If setting the date and time causes any outstanding timers to pass their deadline, then the expired timers will be fired during the invocation of the `rtems_clock_set` directive.

### 9.3.3. Obtaining the Time

RTEMS provides multiple directives which can be used by an application to obtain the current date and time or date and time related information. These directives allow a task or an ISR to obtain the current date and time or date and time related information. The current date and time can be returned in either native or UNIX-style format. Additionally, the application can obtain date and time related information such as the number of seconds since the RTEMS epoch, the number of ticks since the executive was initialized, and the number of ticks per second. The following directives are available:

`rtems_clock_get_tod`

  obtain native style date and time

`rtems_clock_get_time_value`

  obtain UNIX-style date and time

`rtems_clock_get_ticks_since_boot`

  obtain number of ticks since RTEMS was initialized

`rtems_clock_get_seconds_since_epoch`

  obtain number of seconds since RTEMS epoch

`rtems_clock_get_ticks_per_second`

  obtain number of clock ticks per second

Calendar time operations will return an error code if invoked before the date and time have been set.

### 9.3.4. Transition Advice for the Removed rtems_clock_get()

The directive CLOCK_GET - Get date and time information took an untyped pointer with an options argument to indicate the time information desired. This has been replaced with a set of typed directives:

- CLOCK_GET_SECONDS_SINCE_EPOCH - Get seconds since epoch

- CLOCK_GET_TICKS_PER_SECOND - Get ticks per second

- CLOCK_GET_TICKS_SINCE_BOOT - Get current ticks counter value

- CLOCK_GET_TOD - Get date and time in TOD format

- CLOCK_GET_TOD_TIMEVAL - Get date and time in timeval format

These directives directly correspond to what were previously referred to as clock options. These strongly typed directives were available for multiple releases in parallel with `rtems_clock_get()` until that directive was removed.

## 9.4. Directives

This section details the clock manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

### 9.4.1. CLOCK_SET - Set date and time

CALLING SEQUENCE:

      rtems_status_code rtems_clock_set(
          rtems_time_of_day *time_buffer
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | date and time set successfully
  `RTEMS_INVALID_ADDRESS` | `time_buffer` is NULL
  `RTEMS_INVALID_CLOCK` | invalid time of day

DESCRIPTION:

  This directive sets the system date and time. The date, time, and ticks in the time_buffer structure are all range-checked, and an error is returned if any one is out of its valid range.

NOTES:

  Years before 1988 are invalid.

  The system date and time are based on the configured tick rate (number of microseconds in a tick).

  Setting the time forward may cause a higher priority task, blocked waiting on a specific time, to be made ready. In this case, the calling task will be preempted after the next clock tick.

  Re-initializing RTEMS causes the system date and time to be reset to an uninitialized state. Another call to `rtems_clock_set` is required to re-initialize the system date and time to application specific specifications.

### 9.4.2. CLOCK_GET_TOD - Get date and time in TOD format

CALLING SEQUENCE:

      rtems_status_code rtems_clock_get_tod(
          rtems_time_of_day *time_buffer
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | current time obtained successfully
  `RTEMS_NOT_DEFINED` | system date and time is not set
  `RTEMS_INVALID_ADDRESS` | `time_buffer` is NULL

DESCRIPTION:

  This directive obtains the system date and time. If the date and time has not been set with a previous call to `rtems_clock_set`, then the `RTEMS_NOT_DEFINED` status code is returned.

NOTES:

  This directive is callable from an ISR.

  This directive will not cause the running task to be preempted. Re-initializing RTEMS causes the system date and time to be reset to an uninitialized state. Another call to `rtems_clock_set` is required to re-initialize the system date and time to application specific specifications.

### 9.4.3. CLOCK_GET_TOD_TIMEVAL - Get date and time in timeval format

CALLING SEQUENCE:

      rtems_status_code rtems_clock_get_tod_interval(
          struct timeval  *time
      );

DIRECTIVE STATUS CODES:
DESCRIPTION:

  This directive obtains the system date and time in POSIX `struct timeval` format. If the date and time has not been set with a previous call to `rtems_clock_set`, then the `RTEMS_NOT_DEFINED` status code is returned.

NOTES:

  This directive is callable from an ISR.

  This directive will not cause the running task to be preempted. Re-initializing RTEMS causes the system date and time to be reset to an uninitialized state. Another call to `rtems_clock_set` is required to re-initialize the system date and time to application specific specifications.

### 9.4.4. CLOCK_GET_SECONDS_SINCE_EPOCH - Get seconds since epoch

CALLING SEQUENCE:

      rtems_status_code rtems_clock_get_seconds_since_epoch(
          rtems_interval *the_interval
      );

DIRECTIVE STATUS CODES:
DESCRIPTION:

  This directive returns the number of seconds since the RTEMS epoch and the current system date and time. If the date and time has not been set with a previous call to `rtems_clock_set`, then the `RTEMS_NOT_DEFINED` status code is returned.

NOTES:

  This directive is callable from an ISR.

  This directive will not cause the running task to be preempted. Re-initializing RTEMS causes the system date and time to be reset to an uninitialized state. Another call to `rtems_clock_set` is required to re-initialize the system date and time to application specific specifications.

### 9.4.5. CLOCK_GET_TICKS_PER_SECOND - Get ticks per second

CALLING SEQUENCE:

      rtems_interval rtems_clock_get_ticks_per_second(void);

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  This directive returns the number of clock ticks per second. This is strictly based upon the microseconds per clock tick that the application has configured.

NOTES:

  This directive is callable from an ISR.

  This directive will not cause the running task to be preempted.

### 9.4.6. CLOCK_GET_TICKS_SINCE_BOOT - Get current ticks counter value

CALLING SEQUENCE:

      rtems_interval rtems_clock_get_ticks_since_boot(void);

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

> This directive returns the current tick counter value. With a 1ms clock tick, this counter overflows after 50 days since boot. This is the historical measure of uptime in an RTEMS system. The newer service `rtems_clock_get_uptime` is another and potentially more accurate way of obtaining similar information.

NOTES:

> This directive is callable from an ISR.

> This directive will not cause the running task to be preempted.

### 9.4.7. CLOCK_TICK_LATER - Get tick value in the future

CALLING SEQUENCE:

      rtems_interval rtems_clock_tick_later(
          rtems_interval delta
      );

DESCRIPTION:

  Returns the ticks counter value delta ticks in the future.

NOTES:

  This directive is callable from an ISR.

  This directive will not cause the running task to be preempted.

### 9.4.8. CLOCK_TICK_LATER_USEC - Get tick value in the future in microseconds

CALLING SEQUENCE:

      rtems_interval rtems_clock_tick_later_usec(
          rtems_interval delta_in_usec
      );

DESCRIPTION:

  Returns the ticks counter value at least delta microseconds in the future.

NOTES:

  This directive is callable from an ISR.

  This directive will not cause the running task to be preempted.

### 9.4.9. CLOCK_TICK_BEFORE - Is tick value is before a point in time

CALLING SEQUENCE:

      rtems_interval rtems_clock_tick_before(
          rtems_interval tick
      );

DESCRIPTION:

  Returns true if the current ticks counter value indicates a time before the time specified by the tick value and false otherwise.

NOTES:

  This directive is callable from an ISR.

  This directive will not cause the running task to be preempted.

EXAMPLE:

      status busy( void )
      {
          rtems_interval timeout = rtems_clock_tick_later_usec( 10000 );
          do {
              if ( ok() ) {
                  return success;
              }
          } while ( rtems_clock_tick_before( timeout ) );
          return timeout;
      }

### 9.4.10. CLOCK_GET_UPTIME - Get the time since boot

CALLING SEQUENCE:

      rtems_status_code rtems_clock_get_uptime(
          struct timespec *uptime
      );

DIRECTIVE STATUS CODES:
DESCRIPTION:

  This directive returns the seconds and nanoseconds since the system was booted. If the BSP supports nanosecond clock accuracy, the time reported will probably be different on every call.

NOTES:

  This directive may be called from an ISR.

### 9.4.11. CLOCK_GET_UPTIME_TIMEVAL - Get the time since boot in timeval format

CALLING SEQUENCE:

      void rtems_clock_get_uptime_timeval(
          struct timeval *uptime
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  This directive returns the seconds and microseconds since the system was booted. If the BSP supports nanosecond clock accuracy, the time reported will probably be different on every call.

NOTES:

  This directive may be called from an ISR.

### 9.4.12. CLOCK_GET_UPTIME_SECONDS - Get the seconds since boot

CALLING SEQUENCE:

      time_t rtems_clock_get_uptime_seconds(void);

DIRECTIVE STATUS CODES:

  The system uptime in seconds.

DESCRIPTION:

  This directive returns the seconds since the system was booted.

NOTES:

  This directive may be called from an ISR.

### 9.4.13. CLOCK_GET_UPTIME_NANOSECONDS - Get the nanoseconds since boot

CALLING SEQUENCE:

      uint64_t rtems_clock_get_uptime_nanoseconds(void);

DIRECTIVE STATUS CODES:

  The system uptime in nanoseconds.

DESCRIPTION:

  This directive returns the nanoseconds since the system was booted.

NOTES:

  This directive may be called from an ISR.

## 9.5. Removed Directives

### 9.5.1. CLOCK_GET - Get date and time information

Warning

This directive was removed in RTEMS 5.1. See also Transition Advice for the Removed rtems_clock_get().

CALLING SEQUENCE:

      rtems_status_code rtems_clock_get(
         rtems_clock_get_options  option,
         void                    *time_buffer
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | current time obtained successfully
  `RTEMS_NOT_DEFINED` | system date and time is not set
  `RTEMS_INVALID_ADDRESS` | `time_buffer` is NULL

DESCRIPTION:

  This directive obtains the system date and time. If the caller is attempting to obtain the date and time (i.e. option is set to either `RTEMS_CLOCK_GET_SECONDS_SINCE_EPOCH`, `RTEMS_CLOCK_GET_TOD`, or `RTEMS_CLOCK_GET_TIME_VALUE`) and the date and time has not been set with a previous call to `rtems_clock_set`, then the `RTEMS_NOT_DEFINED` status code is returned. The caller can always obtain the number of ticks per second (option is `RTEMS_CLOCK_GET_TICKS_PER_SECOND`) and the number of ticks since the executive was initialized option is `RTEMS_CLOCK_GET_TICKS_SINCE_BOOT`).

  The `option` argument may taken on any value of the enumerated type `rtems_clock_get_options`. The data type expected for `time_buffer` is based on the value of `option` as indicated below:

  Option | Return type
  --- | ---
  `RTEMS_CLOCK_GET_TOD` | `(rtems_time_of_day *)`
  `RTEMS_CLOCK_GET_SECONDS_SINCE_EPOCH` | `(rtems_interval *)`
  `RTEMS_CLOCK_GET_TICKS_SINCE_BOOT` | `(rtems_interval *)`
  `RTEMS_CLOCK_GET_TICKS_PER_SECOND` | `(rtems_interval *)`
  `RTEMS_CLOCK_GET_TIME_VALUE` | `(struct timeval *)`

NOTES:

  This directive is callable from an ISR.

  This directive will not cause the running task to be preempted. Re-initializing RTEMS causes the system date and time to be reset to an uninitialized state. Another call to `rtems_clock_set` is required to re-initialize the system date and time to application specific specifications.