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ALARM(3P)                  POSIX Programmer's Manual                 ALARM(3P)

       This manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may dif-
       fer (consult the corresponding Linux manual page for details of Linux behavior), or the interface  may  not  be
       implemented on Linux.

       alarm - schedule an alarm signal

       #include <unistd.h>

       unsigned alarm(unsigned seconds);

       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 generated, 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.

       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.

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

       The following sections are informative.


       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 considered, but historical imple-
       mentations, 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.

       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 appropriate, since it gives the most predictable behavior,
       especially since the signal can always be delayed for an indefinite amount of time due to scheduling.  Applica-
       tions  can  thus  choose the seconds argument as the minimum amount of time they wish to have elapse before the

       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.


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

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The original Standard can be
       obtained online at .

IEEE/The Open Group                  2003                            ALARM(3P)