ALARM(P)						     POSIX Programmer's Manual							  ALARM(P)

NAME

       alarm - schedule an alarm signal

SYNOPSIS

       #include <unistd.h>

       unsigned alarm(unsigned seconds);

DESCRIPTION

       The  alarm() function shall cause the system to generate a SIGALRM signal for the process after the number of realtime seconds specified by
       seconds have elapsed. Processor scheduling delays may prevent the process from handling the signal as soon as it is generated.

       If seconds is 0, a pending alarm request, if any, is canceled.

       Alarm requests are not stacked; only one SIGALRM generation can be scheduled in this manner. If the SIGALRM signal has not yet been  gener-
       ated, the call shall result in rescheduling the time at which the SIGALRM signal is generated.

       Interactions between alarm() and any of setitimer(), ualarm(), or usleep() are unspecified.

RETURN VALUE

       If  there  is a previous alarm() request with time remaining, alarm() shall return a non-zero value that is the number of seconds until the
       previous request would have generated a SIGALRM signal. Otherwise, alarm() shall return 0.

ERRORS

       The alarm() function is always successful, and no return value is reserved to indicate an error.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The fork() function clears pending alarms in the child process.	A new process image created by one of the exec functions inherits the time
       left to an alarm signal in the old process' image.

       Application  writers  should  note  that  the  type  of the argument seconds and the return value of alarm() is unsigned. That means that a
       Strictly Conforming POSIX System Interfaces Application cannot pass a value greater than the minimum guaranteed value for {UINT_MAX}, which
       the  ISO C  standard sets as 65535, and any application passing a larger value is restricting its portability. A different type was consid-
       ered, but historical implementations, including those with a 16-bit int type, consistently use either unsigned or int.

       Application writers should be aware of possible interactions when the same process uses both the alarm() and sleep() functions.

RATIONALE

       Many historical implementations (including Version 7 and System V) allow an alarm to occur up to  a  second  early.  Other  implementations
       allow  alarms  up to half a second or one clock tick early or do not allow them to occur early at all. The latter is considered most appro-
       priate, since it gives the most predictable behavior, especially since the signal can always be delayed for an indefinite  amount  of  time
       due to scheduling. Applications can thus choose the seconds argument as the minimum amount of time they wish to have elapse before the sig-
       nal.

       The term "realtime" here and elsewhere ( sleep(), times()) is intended to mean "wall clock" time as common English usage, and  has  nothing
       to do with "realtime operating systems". It is in contrast to virtual time, which could be misinterpreted if just time were used.

       In  some  implementations,  including  4.3  BSD,  very large values of the seconds argument are silently rounded down to an implementation-
       defined maximum value. This maximum is large enough (to the order of several months) that the effect is not noticeable.

       There were two possible choices for alarm generation in multi-threaded applications: generation for the calling thread  or  generation  for
       the  process.  The  first option would not have been particularly useful since the alarm state is maintained on a per-process basis and the
       alarm that is established by the last invocation of alarm() is the only one that would be active.

       Furthermore, allowing generation of an asynchronous signal for a thread would have introduced an exception to  the  overall  signal  model.
       This requires a compelling reason in order to be justified.

FUTURE DIRECTIONS

       None.

SEE ALSO

       alarm  ,  exec()  ,  fork()  ,  getitimer()  ,  pause()	,  sigaction()	,  sleep()  , ualarm() , usleep() , the Base Definitions volume of
       IEEE Std 1003.1-2001, <signal.h>, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and  the  original  IEEE
       and  The  Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003								  ALARM(P)