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



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

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
       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
       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 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
       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
       None.

SEE ALSO
       alarm(), exec, fork(), getitimer(), pause(), sigaction(), sleep()

       The Base Definitions volume of POSIX.1-2017, <signal.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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                  2017                             ALARM(3)
