The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

#### NAME

> usleep - suspend execution for an interval

#### SYNOPSIS

> ``[OB XSI] #include <unistd.h>
> int usleep(useconds_t` useconds`);``
> `
> `

#### DESCRIPTION

> The usleep() function shall cause the calling thread to be suspended from execution until either the number of realtime microseconds specified by the argument useconds 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. The suspension time may be longer than requested due to the scheduling of other activity by the system.

> The useconds argument shall be less than one million. If the value of useconds is 0, then the call has no effect.

> If a SIGALRM signal is generated for the calling process during execution of usleep() and if the SIGALRM signal is being ignored or blocked from delivery, it is unspecified whether usleep() returns when the SIGALRM signal is scheduled. If the signal is being blocked, it is also unspecified whether it remains pending after usleep() returns or it is discarded.

> If a SIGALRM signal is generated for the calling process during execution of usleep(), except as a result of a prior call to alarm(), and if the SIGALRM signal is not being ignored or blocked from delivery, it is unspecified whether that signal has any effect other than causing usleep() to return.

> If a signal-catching function interrupts usleep() and examines or changes either the time a SIGALRM is scheduled to be generated, the action associated with the SIGALRM signal, or whether the SIGALRM signal is blocked from delivery, the results are unspecified.

> If a signal-catching function interrupts usleep() and calls siglongjmp() or longjmp() to restore an environment saved prior to the usleep() call, the action associated with the SIGALRM signal and the time at which a SIGALRM signal is scheduled to be generated are unspecified. It is also unspecified whether the SIGALRM signal is blocked, unless the thread's signal mask is restored as part of the environment.

> Implementations may place limitations on the granularity of timer values. For each interval timer, if the requested timer value requires a finer granularity than the implementation supports, the actual timer value shall be rounded up to the next supported value.

> Interactions between usleep() and any of the following are unspecified:

> >     nanosleep()
> >     setitimer()
> >     timer_create()
> >     timer_delete()
> >     timer_getoverrun()
> >     timer_gettime()
> >     timer_settime()
> >     ualarm()
> >     sleep()

> The usleep() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.

#### RETURN VALUE

> Upon successful completion, usleep() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.

#### ERRORS

> The usleep() function may fail if:

> [EINVAL]
>   The time interval specified one million or more microseconds.

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> Applications are recommended to use nanosleep() if the Timers option is supported, or setitimer(), timer_create(), timer_delete(), timer_getoverrun(), timer_gettime(), or timer_settime() instead of this function.

#### RATIONALE

> None.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> alarm(), getitimer(), nanosleep(), sigaction(), sleep() , timer_create(), timer_delete(), timer_getoverrun(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>

#### CHANGE HISTORY

> First released in Issue 4, Version 2.

#### Issue 5

> Moved from X/OPEN UNIX extension to BASE.

> The DESCRIPTION is changed to indicate that timers are now thread-based rather than process-based.

#### Issue 6

> The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

> This function is marked obsolescent.

> IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/144 is applied, updating the DESCRIPTION from "process' signal mask" to "thread's signal mask", and adding a statement that the usleep() function need not be reentrant.

End of informative text.

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