Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.19 sleep,nanosleep,andclock_nanosleepFunctions 373

10.19 sleep, nanosleep,and clock_nanosleep Functions

We’ve used thesleepfunction in numerous examples throughout the text, and we

showed two flawed implementations of it in Figures 10.7 and 10.8.

#include <unistd.h>

unsigned int sleep(unsigned intseconds);

Returns: 0 or number of unslept seconds

This function causes the calling process to be suspended until either

1. The amount of wall clock time specified bysecondshas elapsed.


2. A signal is caught by the process and the signal handler returns.

As with analarmsignal, the actual return may occur at a time later than requested

because of other system activity.

In case 1, the return value is 0. Whensleepreturns early because of some signal

being caught (case 2), the return value is the number of unslept seconds (the requested

time minus the actual time slept).

Althoughsleepcan be implemented with thealarmfunction (Section 10.10), this

isn’t required. Ifalarmis used, however,therecan be interactions between the two

functions. The POSIX.1 standardleaves all these interactions unspecified. For example,

if we do analarm(10)and 3 wall clock seconds later do asleep(5),what happens?

Thesleepwill return in 5 seconds (assuming that some other signal isn’t caught in the

interim), but will anotherSIGALRMbe generated 2 seconds later? These details depend

on the implementation.

FreeBSD 8.0, Linux 3.2.0, Mac OS X 10.6.8, and Solaris 10 implementsleepusing the

nanosleepfunction, which allows the implementation to be independent of signals and the

alarm timer.For portability,you shouldn’t make any assumptions about the implementation

ofsleep,but if you have any intentions of mixing calls tosleepwith any other timing

functions, you need to be aware of possible interactions.

Example

Figure10.29 shows an implementation of the POSIX.1sleepfunction. This function is

amodification of Figure10.7, which handles signals reliably,avoiding the race condition

in the earlier implementation. We still do not handle any interactions with previously

set alarms. (As we mentioned, these interactions areexplicitly undefined by POSIX.1.)

#include "apue.h"

static void

sig_alrm(int signo)

{

/* nothing to do, just returning wakes up sigsuspend() */

}

unsigned int