Friday, October 28, 2011

sleep function in c

sleep Function

We've used the sleep function in numerous examples throughout the text, and we showed two flawed implementations of it below

#include <unistd.h>

unsigned int sleep(unsigned int seconds);

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 by seconds has elapsed. 
  2. A signal is caught by the process and the signal handler returns.
As with an alarm signal, the actual return may be at a time later than requested, because of other system activity.

In case 1, the return value is 0. When sleep returns 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).

Although sleep can be implemented with the alarm function, this isn't required. If alarm is used, however, there can be interactions between the two functions. The POSIX.1 standard leaves all these interactions unspecified. For example, if we do an alarm(10) and 3 wall clock seconds later do a sleep(5), what happens? The sleep will return in 5 seconds (assuming that some other signal isn't caught in that time), but will another SIGALRM be generated 2 seconds later? These details depend on the implementation.

Solaris 9 implements sleep using alarm. The Solaris sleep(3) manual page says that a
previously scheduled alarm is properly handled. For example, in the preceding scenario, before sleep returns, it will reschedule the alarm to happen 2 seconds later; sleep returns 0 in this case. (Obviously, sleep must save the address of the signal handler for SIGALRM and reset it before returning.) Also, if we do an alarm(6) and 3 wall clock seconds later do a sleep(5), the sleep returns in 3 seconds (when the alarm goes off), not in 5 seconds. Here, the return value from sleep is 2 (the number of unslept seconds).

FreeBSD 5.2.1, Linux 2.4.22, and Mac OS X 10.3, on the other hand, use another technique: the delay is provided by nanosleep(2). This function is specified to be a high-resolution delay by the real-time extensions in the Single UNIX Specification. This function allows the implementation of sleep to be independent of signals.

For portability, you shouldn't make any assumptions about the implementation of sleep, but if you have any intentions of mixing calls to sleep with any other timing functions, you need to be aware of possible interactions.


Figure below shows an implementation of the POSIX.1 sleep function.  This function 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 are explicitly undefined by POSIX.1.)

It takes more code to write this reliable implementation than what is shown in Figure below. We don't use any form of nonlocal branching, so there is no effect on other signal handlers that may be executing when the SIGALRM is handled.

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