PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

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

NAME         top

       alarm — schedule an alarm signal

SYNOPSIS         top

       #include <unistd.h>

       unsigned alarm(unsigned seconds);

DESCRIPTION         top

       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 setitimer() are unspecified.

RETURN VALUE         top

       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         top

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

       The following sections are informative.

EXAMPLES         top

       None.

APPLICATION USAGE         top

       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 image of the old process.

       Application developers 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 implementations, including those with a
       16-bit int type, consistently use either unsigned or int.

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

RATIONALE         top

       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. Applications can thus choose the seconds
       argument as the minimum amount of time they wish to have elapse
       before the signal.

       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-
       specific 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         top

       None.

SEE ALSO         top

       alarm(3p), exec(1p), fork(3p), getitimer(3p), pause(3p),
       sigaction(3p), sleep(3p)

       The Base Definitions volume of POSIX.1‐2008, signal.h(0p),
       unistd.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                           ALARM(3P)