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

# 15. Clock Manager

## 15.1. Introduction

The clock manager provides services two primary classes of services. The first focuses on obtaining and setting the current date and time. The other category of services focus on allowing a thread to delay for a specific length of time.

The directives provided by the clock manager are:

- clock_gettime - Obtain Time of Day

- clock_settime - Set Time of Day

- clock_getres - Get Clock Resolution

- sleep - Delay Process Execution

- usleep - Delay Process Execution in Microseconds

- nanosleep - Delay with High Resolution

- gettimeofday - Get the Time of Day

- time - Get time in seconds

## 15.2. Background

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## 15.3. Operations

There is currently no text in this section.

## 15.4. 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.

### 15.4.1. clock_gettime - Obtain Time of Day

CALLING SEQUENCE:

    #include <time.h>
    int clock_gettime(
        clockid_t        clock_id,
        struct timespec *tp
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` to one of the following:

`EINVAL` | The tp pointer parameter is invalid.
`EINVAL` | The clock_id specified is invalid.

DESCRIPTION:

NOTES:

NONE

### 15.4.2. clock_settime - Set Time of Day

CALLING SEQUENCE:

    #include <time.h>
    int clock_settime(
        clockid_t              clock_id,
        const struct timespec *tp
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` to one of the following:

`EINVAL` | The tp pointer parameter is invalid.
`EINVAL` | The clock_id specified is invalid.
`EINVAL` | The contents of the tp structure are invalid.

DESCRIPTION:

NOTES:

NONE

### 15.4.3. clock_getres - Get Clock Resolution

CALLING SEQUENCE:

    #include <time.h>
    int clock_getres(
        clockid_t        clock_id,
        struct timespec *res
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` to one of the following:

`EINVAL` | The res pointer parameter is invalid.
`EINVAL` | The clock_id specified is invalid.

DESCRIPTION:

NOTES:

If `res` is `NULL`, then the resolution is not returned.

### 15.4.4. sleep - Delay Process Execution

CALLING SEQUENCE:

    #include <unistd.h>
    unsigned int sleep(
        unsigned int seconds
    );

STATUS CODES:

This routine returns the number of unslept seconds.

DESCRIPTION:

The `sleep()` function delays the calling thread by the specified number of `seconds`.

NOTES:

This call is interruptible by a signal.

### 15.4.5. usleep - Delay Process Execution in Microseconds

CALLING SEQUENCE:

    #include <time.h>
    useconds_t usleep(
        useconds_t useconds
    );

STATUS CODES:

This routine returns the number of unslept seconds.

DESCRIPTION:

The `sleep()` function delays the calling thread by the specified number of `seconds`.

The `usleep()` function suspends the calling thread from execution until either the number of microseconds specified by the `useconds` argument has elapsed or a signal is delivered to the calling thread and its action is to invoke a signal-catching function or to terminate the process.

Because of other activity, or because of the time spent in processing the call, the actual length of time the thread is blocked may be longer than the amount of time specified.

NOTES:

This call is interruptible by a signal.

The Single UNIX Specification allows this service to be implemented using the same timer as that used by the `alarm()` service. This is NOT the case for RTEMS and this call has no interaction with the `SIGALRM` signal.

### 15.4.6. nanosleep - Delay with High Resolution

CALLING SEQUENCE:

    #include <time.h>
    int nanosleep(
        const struct timespec *rqtp,
        struct timespec       *rmtp
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` to one of the following:

`EINTR` | The routine was interrupted by a signal.
`EAGAIN` | The requested sleep period specified negative seconds or nanoseconds.
`EINVAL` | The requested sleep period specified an invalid number for the nanoseconds field.

DESCRIPTION:

NOTES:

This call is interruptible by a signal.

### 15.4.7. gettimeofday - Get the Time of Day

CALLING SEQUENCE:

    #include <sys/time.h>
    #include <unistd.h>
    int gettimeofday(
        struct timeval  *tp,
        struct timezone *tzp
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` as appropriate.

`EPERM` | `settimeofdat` is called by someone other than the superuser.
`EINVAL` | Timezone (or something else) is invalid.
`EFAULT` | One of `tv` or `tz` pointed outside your accessible address space

DESCRIPTION:

This routine returns the current time of day in the `tp` structure.

NOTES:

Currently, the timezone information is not supported. The `tzp` argument is ignored.

### 15.4.8. time - Get time in seconds

CALLING SEQUENCE:

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

STATUS CODES:

This routine returns the number of seconds since the Epoch.

DESCRIPTION:

`time` returns the time since 00:00:00 GMT, January 1, 1970, measured in seconds

If `tloc` in non null, the return value is also stored in the memory pointed to by `t`.

NOTES:

NONE